libxl: handle read-only drives with qemu-xen

libxl: handle read-only drives with qemu-xen

The current libxl code doesn't deal with read-only drives at all.

Upstream QEMU and qemu-xen only support read-only cdrom drives: make
sure to specify "readonly=on" for cdrom drives and return error in case
the user requested a non-cdrom read-only drive.

This is XSA-142, discovered by Lin Liu
(https://bugzilla.redhat.com/show_bug.cgi?id=1257893).

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>