xl: fix the 'upgrade' of Ide devices to Xen devices

Without this patch a VM.start will hang in 'wait_for_plug' because
the hotplug script writes the xvda paths (51712) while xenopsd
waits for the hda paths (768)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #142 from djs55/CA-149193

xl: fix the 'upgrade' of Ide devices to Xen devices