hw/virtio: add vhost-user-gpio-pci boilerplate

hw/virtio: add vhost-user-gpio-pci boilerplate

This allows is to instantiate a vhost-user-gpio device as part of a PCI
bus. It is mostly boilerplate which looks pretty similar to the
vhost-user-fs-pci device.

Signed-off-by: Viresh Kumar <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-Id: <5f560cab92d0d789b1c94295ec74b9952907d69d.1641987128[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <20220802095010.3330793[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>