vm.Vm._getUnderlyingDriveInfo: extract path of gluster disks

_getUnderlyingDriveInfo() is broken by design. It has no reliable means
to match Engine-requested devices with libvirt-produced ones. As a
heuristic, it uses the device's as matching key. However, before this
patch, it failed to extract the path from the xml definition of gluster
disks.

Bug-Url: https://bugzilla.redhat.com/1007980
Change-Id: I4459916cab24d735c067a6eb7020d4f43505fc97
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19906
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Martin Polednik <mpoledni@redhat.com>
Tested-by: Martin Polednik <mpoledni@redhat.com>