Having a single QXL interface is not enough, there can be other (e.g.
streaming) display channels that make the tablet unusable. Add a check for the
number of display channels also being equal to 1. We still need the check for
QXL interaces, because the tablet only works with QXL.
Signed-off-by: Lukáš Hrázký <lhrazky@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>