vhost-user-gpu: abstract vg_cleanup_mapping_iov

vhost-user-gpu: abstract vg_cleanup_mapping_iov

Currently in vhost-user-gpu, we free resource directly in
the cleanup case of resource. If we change the cleanup logic
we need to change several places, also abstruct a
'vg_create_mapping_iov' can be symmetry with the
'vg_create_mapping_iov'. This is like what virtio-gpu does,
no function changed.

Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210516030403.107723-9-liq3ea@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>