vhost-user: pass message as a pointer to process_message_reply()

vhost-user: pass message as a pointer to process_message_reply()

process_message_reply() was recently updated to get full message
content instead of only its request field.

There is no need to copy all the struct content into the stack,
so just pass its pointer as const.

Reviewed-by: Jens Freimann <jfreiman@redhat.com>
Reviewed-by: Zhiyong Yang <zhiyong.yang@intel.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>