implement cdrom eject from the guest

implement cdrom eject from the guest

Hi all,
this patch allows a guest to eject the cdrom: when qemu detects that a
cdrom eject request ahs been issued by the guest, it writes eject to the
corresponding xenstore frontend, so that the toolstack can take care of
removing the current cdrom frontend\backend couple and create an empty one
instead.

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