libxenlight: implement cdrom insert/eject

libxenlight: implement cdrom insert/eject

This patch implements functions in libxenlight to change the cdrom in
a VM at run time and to handle cdrom eject requests from guests.

This patch adds two new commands to xl: cd-insert and cd-eject; it
also modifies xl to handle cdrom eject requests coming from guests
(actually coming from qemu).

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