kmscon: add --vt=<num> option to select VT for seat0

kmscon: add --vt=<num> option to select VT for seat0

VTs are only available on seat0 so this option does not affect other
seats. On seat0 it selects the VT that we run on. We do _not_ fall back to
another seat if it fails but disable this seat instead.

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