libxl: allow 'phy' backend to use empty files

libxl: allow 'phy' backend to use empty files

This was introduced by 97ee1f (~5 years ago), but was probably never
surfaced because most people used regular files as CDROM images, so the PHY
backend was actually never selected. A year ago this was changed, and now
regular RAW files are also handled by the PHY backend, which has made this
bug suface.

Fix it by allowing empty disks to use the PHY backend, skipping the stat
tests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Alex Braunegg <alex.braunegg@gmail.com>