Replace 'width' and 'height' in QemuDmaBuf with 'backing_widht'
and 'backing_height' as these commonly indicate the size of the
whole surface (e.g. guest's Xorg extended display). Then use
'width' and 'height' for sub region in there (e.g. guest's
scanouts).

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: <20230713040444.32267-1-dongwon.kim@intel.com>