core: Fix test break in LiveMigrateVmDisksCommand

Change-Id Ifefe823faa8e546eba3e9d34c4b46609085baad8 changed the
semantics of how storage domains were validated, which broke the unit
test.

This patch mocks the storage domain ID on the image correctly, so the
test can continue functioning.

Change-Id: I79162cffafcfd385fdced56e23e6e005828c21f9
Signed-off-by: Allon Mureinik <amureini@redhat.com>