hw/virtio: derive vhost-user-gpio from vhost-user-base

hw/virtio: derive vhost-user-gpio from vhost-user-base

Now the new base class supports config handling we can take advantage
and make vhost-user-gpio a much simpler boilerplate wrapper. Also as
this doesn't require any target specific hacks we only need to build
the stubs once.

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240104210945.1223134-5-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>