blkfront: do not create a PV cdrom device if xen_hvm_guest

It is not possible to unplug emulated cdrom devices, and PV cdroms don't
handle media insert, eject and stream, so we are better off disabling PV
cdroms when running as a Xen HVM guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>