Dell PowerFlex: Size shown for a volume doesn't match powerflex backend allocation

Description

When creating a volume, powerflex will round the value of the capacity by a multiple of 8GB leaving inconsistency between cinder which shows the value requested by the user and powerflex which shows the current allocation.

Step to reproduce

Create a volume with a size different from 8GB multiplier
stack@SRA-Left-Openstack1:~/devstack$ openstack volume create --size 3 --type powerflex test_vol_pflex
/usr/lib/python3/dist-packages/secretstorage/dhcrypto.py:15: CryptographyDeprecationWarning: int_from_bytes is deprecated, use int.from_bytes instead
  from cryptography.utils import int_from_bytes
/usr/lib/python3/dist-packages/secretstorage/util.py:19: CryptographyDeprecationWarning: int_from_bytes is deprecated, use int.from_bytes instead
  from cryptography.utils import int_from_bytes
+---------------------+--------------------------------------+
| Field               | Value                                |
+---------------------+--------------------------------------+
| attachments         | []                                   |
| availability_zone   | nova                                 |
| bootable            | false                                |
| consistencygroup_id | None                                 |
| created_at          | 2022-09-27T09:05:36.735298           |
| description         | None                                 |
| encrypted           | False                                |
| id                  | ebcc7949-2777-4841-9001-6da2b081b6d2 |
| migration_status    | None                                 |
| multiattach         | False                                |
| name                | test_vol_pflex                       |
| properties          |                                      |
| replication_status  | None                                 |
| size                | 3                                    |
| snapshot_id         | None                                 |
| source_volid        | None                                 |
| status              | creating                             |
| type                | powerflex                            |
| updated_at          | None                                 |
| user_id             | cf67ca76a2924d21988a3d21101324dd     |
+---------------------+--------------------------------------+
stack@SRA-Left-Openstack1:~/devstack$ openstack volume list
/usr/lib/python3/dist-packages/secretstorage/dhcrypto.py:15: CryptographyDeprecationWarning: int_from_bytes is deprecated, use int.from_bytes instead
  from cryptography.utils import int_from_bytes
/usr/lib/python3/dist-packages/secretstorage/util.py:19: CryptographyDeprecationWarning: int_from_bytes is deprecated, use int.from_bytes instead
  from cryptography.utils import int_from_bytes
+--------------------------------------+----------------+-----------+------+-------------------------------------+
| ID                                   | Name           | Status    | Size | Attached to                         |
+--------------------------------------+----------------+-----------+------+-------------------------------------+
| ebcc7949-2777-4841-9001-6da2b081b6d2 | test_vol_pflex | available |    3 |                                     |
| 048a3748-8517-48df-9d37-e16ec2789408 | vol_pflex4     | in-use    |    8 | Attached to server1 on /dev/vdb     |
| 470839d8-bfcf-45c5-ba9f-e6608c42bc49 | voltosnap      | available |    9 |                                     |
| 620b2198-47bc-4d2a-9a60-2df4c35e91f4 |                | in-use    |    9 | Attached to serversnap on /dev/vda  |
| 24e11e0b-44ad-4bee-9dc9-09e64f9413a1 | vol_pflex3     | available |    9 |                                     |
| aadf3632-8419-47a2-bbe7-950b51f93fae | vol_pflex3     | error     |    8 |                                     |
| 4c6092bd-75c0-4546-9360-e3e0ef224779 | vol_pflex2     | in-use    |    8 | Attached to server1 on /dev/vda     |
| 80e0751c-4849-4981-a7cc-2b98d7d360ea | vol_pflex      | available |   10 |                                     |
+--------------------------------------+----------------+-----------+------+-------------------------------------+

This introduces multiple side effects:

1. Allocation capacity not being monitored correctly
2. Extension of volume withing the same range will fail as the volume is already at that size (Example: volume extenstion from 5 to 7, backend already set to 8)
3. Quota also affected

This bug consolidates other bugs opened for various behavior but still happens because of the same issue:
https://bugs.launchpad.net/cinder/+bug/1915015
https://bugs.launchpad.net/cinder/+bug/1935028
https://bugs.launchpad.net/cinder/+bug/1952355