core: fix possible NPE in internal migration

The check for migration support uses the VM and it was executed before
the VM was validated (that it is not null). Thus, this patch changes the
order of the checks, so that the migration support check will be
executed after the checks in MigrateVmCommand are executed.

Change-Id: I1c287d989e62daa43e834d9f9657dac9820764a3
Signed-off-by: Arik Hadas <ahadas@redhat.com>