I'm not sure why our code claimed "-boot menu=on" cannot be used in
combination with per-device bootindex, but it was proved wrong about
four years ago by commit 8c952908. Let's always use bootindex when QEMU
supports it.

https://bugzilla.redhat.com/show_bug.cgi?id=1323085Signed-off-by: NJiri Denemark <jdenemar@redhat.com>