[pod, wishlist] Add support for multiple storage pools.

I have configured a Pod using Virsh and it is only showing a Local storage (118.9 GiB) however I use a ZFS pool that is 14TB.

virsh pool-list
 Name State Autostart
-------------------------------------------
 default active no
 zfspool active yes

virsh pool-info default
Name: default
UUID: a33198cd-afb7-4184-9b2e-f18fb30bc72f
State: running
Persistent: yes
Autostart: no
Capacity: 102.61 GiB
Allocation: 22.17 GiB
Available: 80.45 GiB

virsh pool-info zfspool
Name: zfspool
UUID: f64a089b-1485-47d2-bf3d-bee5a18af823
State: running
Persistent: yes
Autostart: yes
Capacity: 16.25 TiB
Allocation: 9.48 GiB
Available: 16.24 TiB

maas maas pod read 1
Success.
Machine-readable output follows:
{
    "total": {
        "cores": 24,
        "local_storage": 127624953200,
        "memory": 128907
    },
    "resource_uri": "/MAAS/api/2.0/pods/1/",
    "name": "iaasdev",
    "capabilities": [
        "composable",
        "dynamic_local_storage",
        "over_commit"
    ],
    "type": "virsh",
    "available": {
        "cores": 24,
        "local_storage": 127624953200,
        "memory": 128907
    },
    "used": {
        "cores": 0,
        "local_storage": 0,
        "memory": 0
    },
    "architectures": [
        "amd64/generic"
    ],
    "id": 1
}