vhost-user-gpu: Move QOM macro to header

vhost-user-gpu: Move QOM macro to header

Move the VHOST_USER_GPU type checking macro to virtio-gpu.h,
close to the TYPE_VHOST_USER_GPU #define.

This will make future conversion to OBJECT_DECLARE* easier.

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-By: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200825192110.3528606-30-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>