virgl: wrap vertex element state in a struct

This just has one member for now; the handle. But this is about to
change.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Tested-By: Gert Wollny <gert.wollny@collabora.com>
(cherry picked from commit 8447b642)