virtio-gpu: update done only on the scanout associated with rect

It only needs to update the scanouts containing the rect area
coming with the resource-flush request from the guest.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20220505214030.4261-1-dongwon.kim@intel.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>