core: fix possible NPE in migrate vm to server

Rearrange the order of the checks which are carried out in
MigrationVmToServerCommand#canDoAction method, such that checks which
are use the VM will be executed after the VM is validated in
MigrateVmCommand#canDoAction.

Change-Id: Icc96ecb14eb0350823dc7ea2fd95a2b56ca2ec41
Signed-off-by: Arik Hadas <ahadas@redhat.com>