vl: preconfig and loadvm are mutually exclusive

vl: preconfig and loadvm are mutually exclusive

Just like -incoming.  Later we will add support for "-incoming defer
-preconfig", because there are cases (Xen, block layer) that want
to look at RUNSTATE_INMIGRATE.  -loadvm will remain mutually exclusive
with preconfig; the plan is to just do loadvm in the monitor, since
the user is already going to interact with it for preconfiguration.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>