libvirt: skip disk resize when resize_instance is False

The method finish_migration is called with an extra
parameter 'resize_instance'. It should be used to know
if it is necessary or not to resize the disks and so
call the method 'disk_resize'.

Change-Id: Ia5a9ad7994fa9abab7cf47d36468c46d97065991
Closes-Bug: #1298981
Merge "libvirt: skip disk resize when resize_instance is False"