virsh: don't print --(null) in vol-name and vol-pool

Don't print the pool option name if it's null.

Before:
virsh # vol-name vol
error: failed to get vol 'vol', specifying --(null) might help
error: Storage volume not found: no storage vol with matching path vol

After:

virsh # vol-name vol
error: failed to get vol 'vol'
error: Storage volume not found: no storage vol with matching path vol

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=924571

tools/virsh-volume.c

@@ -88,7 +88,7 @@ vshCommandOptVolBy(vshControl *ctl, const vshCmd *cmd,
     }
 
     if (!vol) {
-        if (pool)
+        if (pool || !pooloptname)
             vshError(ctl, _("failed to get vol '%s'"), n);
         else
             vshError(ctl, _("failed to get vol '%s', specifying --%s "