core: MigrateVmDisks - get source domain by disk

LiveMigrateVmDisksCommand - source storage domain should
be retrieved from the operated disk instead of the command parameters
(only a disk that resides on a single storage domain can be moved).

Change-Id: Ifefe823faa8e546eba3e9d34c4b46609085baad8
Bug-Url: https://bugzilla.redhat.com/883871
Signed-off-by: Daniel Erez <derez@redhat.com>