CA-141526: Putting VBD path to /dev/xvdX for HVM guests

/dev/hda is anyways meaningless for HVM guest.So, we always populate the
VBD path to be /dev/xvdX.

Imported from xen-api.git fc5108344c12a735398a050ae39b6ddf39d98dd0

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>
Imported-by: John Else <john.else@citrix.com>
Merge pull request #35 from johnelse/ca-141526-master

CA-141526: Putting VBD path to /dev/xvdX for HVM guests