Support teardownVolumePath(None)

Change-Id: I68bc4996ec1789e090111e8aeadaf733e163543b
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=962549
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15316
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>