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>

tools/libxl/libxl_device.c

@@ -191,20 +191,13 @@ static int disk_try_backend(disk_try_backend_args *a,
 
     switch (backend) {
     case LIBXL_DISK_BACKEND_PHY:
-        if (!(a->disk->format == LIBXL_DISK_FORMAT_RAW ||
-              a->disk->format == LIBXL_DISK_FORMAT_EMPTY)) {
+        if (a->disk->format != LIBXL_DISK_FORMAT_RAW) {
             goto bad_format;
         }
 
         if (libxl_defbool_val(a->disk->colo_enable))
             goto bad_colo;
 
-        if (a->disk->format == LIBXL_DISK_FORMAT_EMPTY) {
-            LOG(DEBUG, "Disk vdev=%s is empty, skipping physical device check",
-                a->disk->vdev);
-            return backend;
-        }
-
         if (a->disk->backend_domid != LIBXL_TOOLSTACK_DOMID) {
             LOG(DEBUG, "Disk vdev=%s, is using a storage driver domain, "
                        "skipping physical device check", a->disk->vdev);