restapi: Start VM not as RunOnce

The BackendVmResource.start issued RunVmOnce command even if none
additional parameters were provided.

Now we check if a Vm is passed in the Action and if not, we issue
regular RunVmCommand.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1005562
Change-Id: I40be200c8742d8b31b7881926c59deaf345992ec
Signed-off-by: Martin Betak <mbetak@redhat.com>