hw/virtio: add boilerplate for vhost-user-gpio device

hw/virtio: add boilerplate for vhost-user-gpio device

This creates the QEMU side of the vhost-user-gpio device which connects
to the remote daemon. It is based of vhost-user-i2c code.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <5390324a748194a21bc99b1538e19761a8c64092.1641987128.git.viresh.kumar@linaro.org>
[AJB: fixes for qtest, tweaks to feature bits]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Message-Id: <20220802095010.3330793-13-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>