Set vc->gfx.guest_fb.dmabuf to NULL to prevent any further access
to it after the dmabuf is released.

v2: move declaration of vc inside ifdef

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230627005316.5627-1-dongwon.kim@intel.com>