virsh: Clear vsh last error during virshCommandOptVolBy

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: NJohn Ferlan <jferlan@redhat.com>

tools/virsh-volume.c

@@ -129,6 +129,8 @@ virshCommandOptVolBy(vshControl *ctl, const vshCmd *cmd,
         else
             vshError(ctl, _("failed to get vol '%s', specifying --%s "
                             "might help"), n, pooloptname);
+    } else {
+        vshResetLibvirtError();
     }
 
     /* If the pool was specified, then make sure that the returned