CA-114333: Allow Linux HVM guests to use >4 devices.

Windows PV drivers don't check the guest device name, but the Linux kernel does
With devices > 4, Linux expects them to be in xvdN format, so this patch ensures
that all devices are treated as Linux devices

Signed-off-by: Bob Ball <bob.ball@citrix.com>

Conflicts:

	ocaml/xenops/device_number.ml
Merge pull request #1452 from BobBall/device_numbering

CA-114333: Allow Linux HVM guests to use >4 devices.