core: minor cleanup in MigrateVDSCommand

Change-Id: I55b4365d9a71a47b4a72f98bae649a363f43ef03
Bug-Url: https://bugzilla.redhat.com/1118187
Signed-off-by: Arik Hadas <ahadas@redhat.com>
core: make migrate commands non-transactive

Patch 94e48a02 removed the global transaction during RunVmCommand.
Global transactions should also be removed from the migrate commands
for the same reasons and because it prevents us from migrating multiple
VMs from different hosts to the same host while one of the migrations
is in its execute phase or start a VM on a host while another VM is
migrating to that host and the migration is in its execute phase.

Change-Id: I4894ebd5f062174f9780926a0ce6b499fcc0fc7b
Bug-Url: https://bugzilla.redhat.com/1118187
Signed-off-by: Arik Hadas <ahadas@redhat.com>