vl: Table-based select_vgahw()

Instead of implementing separate check functions for each vga
interface type, add a table enumerating the possible VGA
interfaces.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
