hw/virtio: add config support to vhost-user-device

hw/virtio: add config support to vhost-user-device

To use the generic device the user will need to provide the config
region size via the command line. We also add a notifier so the guest
can be pinged if the remote daemon updates the config.

With these changes:

  -device vhost-user-device-pci,virtio-id=41,num_vqs=2,config_size=8

is equivalent to:

  -device vhost-user-gpio-pci

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230710153522.3469097-11-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>