https://bugzilla.redhat.com/show_bug.cgi?id=1529256
If one of the virStorageVolLookupBy{Key|Name|Path} succeeds and
we have a @vol, then clear the last libvirt error; otherwise, a
subsequent "other" failure may cause vshReportError to erroneously
report the wrong error as well as a reported 'vshError' error that
caused the failure.
Signed-off-by: John Ferlan <jferlan@redhat.com>