qtest: add display-vga-test to ppc64

Only enable for ppc64 in the Makefile, but added
code in the file to check cirrus card only on architectures
supporting it (alpha, mips, i386, x86_64).

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
