[enhacement] Create and pass serial or wwn to VM block devices created via virsh pods

When curtin gets support for maas-devicename -> serial/wwn-based symlinks it will become important to actually pass persistent identifiers to the hypervisor via MAAS, see

https://bugs.launchpad.net/curtin/+bug/1735839
https://bugs.launchpad.net/curtin/+bug/1735839/comments/10

For virtio-blk devices this would mean using <serial> element in a domain xml file:

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/mnt/libvirt/images/maas.qcow2'/>
      <backingStore/>
      <target dev='sda' bus='scsi'/>
      <serial>a76f8801-92d3-4aff-9c96-a51a054517a0</serial> # <-- this
      <alias name='scsi0-0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

# QEMU CLI arg
-drive file=/var/lib/libvirt/images/maas-1.qcow2,format=qcow2,if=none,id=drive-virtio-disk0,serial=a76f8801-92d3-4aff-9c96-a51a054517a0 # ...

cat /sys/class/block/vda/serial
a76f8801-92d3-4aff-9

A UUID will be truncated as a serial number has a limited length.

For virtio-scsi devices

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/mnt/libvirt/images/maas.qcow2'/>
      <backingStore/>
      <target dev='sda' bus='scsi'/>
      <wwn>5000c50015ea71ad</wwn> # <--- this
      <alias name='scsi0-0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <alias name='scsi0'/>
      <address type='pci' domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
    </controller>

# QEMU CLI arg
-device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=1,wwn=0x5000c50015ea71ad

by-id symlinks:

├── wwn-0x5000c50015ea71ad -> ../../sda
├── wwn-0x5000c50015ea71ad-part1 -> ../../sda1
└── wwn-0x5000c50015ea71ad-part2 -> ../../sda2

Relevant QEMU and libvirt commits:

https://git.qemu.org/?p=qemu.git;a=commit;h=95ebda85e09ed2b7f00deb2adbdafa5ccf5db948
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=64cc22841e72d37d577416f5836155ecd0a9bfb6
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=9a502563eef7d7c2c9120b237059426e229eefe9
https://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=6e73850b01ee7d5816a803d684e9d669dad036f3