libxl: Cleanup: Have libxl__alloc_vdev use /libxl

libxl: Cleanup: Have libxl__alloc_vdev use /libxl

When allocating a vdev for a new disk, look in /libxl/device, rather
than the frontends directory in xenstore.

This is more in line with the other parts of libxl, which ought not to
trust frontends.  In this case, though, there is no security bug prior
to this patch because the frontend is the toolstack domain itself.

If libxl__alloc_vdev were ever changed to take a frontend domain
argument, this patch will fix a latent security bug.

This is a followup to XSA-175.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>