core: change the order of checks in canDoAction of migrate vm

Change the order in which the checks whether a snapshot is being taken
for the VM or whether one of its disks is locked are carried on, so that
the snapshot check will be made before, as in that case the disks of the
VM are also locked but this check produces a more specific can-do-action
error for the user.

Change-Id: I3e7cacc97fddd48e16564fda0911e7c9604fc731
Signed-off-by: Arik Hadas <ahadas@redhat.com>