conf: print warning if --vt is used wrong

conf: print warning if --vt is used wrong

--vt should only be used when kmscon runs on a single seat. Otherwise,
this VT will be forced on all seats and each kmscon instance _might_
collide.
If this is done with care, the --vt/--seats combination can work. However,
the use-cases are negligible and we should rather warn the ordinary user
not to do it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>