libxl: remove code added to use the 'phy' backend with CDROM devices

libxl: remove code added to use the 'phy' backend with CDROM devices

This is a partial revert of 612f15, that allowed CDROM devices to use the
'phy' PV backend. Due to limitations in the current implementation of the
libxl_cdrom_insert function, the PV backend used in conjunction with an
emulated CDROM device must always be Qdisk at the moment. This is due to
libxl_cdrom_insert not running disk hotplug scripts on plug and unplug of PV
CDROM backends (and possibly other yet to be identified issues).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>