Added serial attribute for disks in VM

[jfeflinski]

Description

Sometimes I need to have unique serials for disk in the VM - serials are created from the VM ID and next number, e.g.:

7771-1
7771-2
7771-3
7771-4
.
.
.
7771-N

Branches to which this PR applies

• master
• one-X.X

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• Check this if this PR should not be squashed

[rsmontero]

Self-note before merging, from libvirt documentation:

If present, this specify serial number of virtual hard drive. For example, it may look like <serial>WD-WMAP9A966149</serial>. Not supported for scsi-block devices, that is those using disk type 'block' using device 'lun' on bus 'scsi'. Since 0.7.1

• Format, length is probably not a problem
• Type, we need to check disk is not of type block