Setup destination disk from virt_disk_size

When running live-migration --block-migrate on a qcow2 backed
VM without cow image, the destination qcow2 file should be created
with the virtual disk size. For raw images, the virt_disk_size
is set to disk_size to ensure that virt_disk_size is always the
size of the disk that should be re-created.

Update unit tests to be more strict and check for sizes to be correct.

Closes-Bug: #1257355

Change-Id: Ie3be46024f06b9f59af92f5e3918a1958386d4f1
Merge "Setup destination disk from virt_disk_size" into stable/havana