libvirt: split and test finish_migration disk resize

Adds a '_disk_resize' method to help resizing disk
by checking and preparing the disk to be extend with disk.api.
Update finish_migration to use this helper.

Closes-Bug: #1298975
Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>

Change-Id: Iefe16915a2e657d848629d7e4a4fcf67298f913e
Merge "libvirt: split and test finish_migration disk resize"