libxl: Set disk format for empty cdrom device

libxl: Set disk format for empty cdrom device

The XML config for a CDROM device can be without a source path,
indicating that there is no media present. Without this change
the libxl driver fails to start a guest in that case because
the libxl library checks for the LIBXL_DISK_FORMAT_EMPTY format
type and tries to stat the NULL pointer that gets passed on.

> libxl: error: libxl_device.c:265:libxl__device_disk_set_backend:
> Disk vdev=hdc failed to stat: (null): Bad address

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>