chr-testdev: Silently fail init

If there's no virtio-console / chr-testdev configured, then the user
probably didn't want them. Just silently fail rather than stating
the obvious.

Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>