daemon: Preface polkit error output with 'polkit:'

There's been a few bugs about an expected error from polkit:

https://bugzilla.redhat.com/show_bug.cgi?id=873799
https://bugzilla.redhat.com/show_bug.cgi?id=872166

The error is:

Authorization requires authentication but no agent is available.

The error means that polkit needs a password, but there is no polkit
agent registered in your session. Polkit agents are the bit of UI that
pop up and actually ask for your password.

Preface the error with the string 'polkit:' so folks can hopefully
make more sense of it.

daemon/remote.c

@@ -2887,9 +2887,10 @@ error:
     if (authdismissed) {
         virReportError(VIR_ERR_AUTH_CANCELLED, "%s",
                        _("authentication cancelled by user"));
+    } else if (pkout && *pkout) {
+        virReportError(VIR_ERR_AUTH_FAILED, _("polkit: %s"), pkout);
     } else {
-        virReportError(VIR_ERR_AUTH_FAILED, "%s",
-                       pkout && *pkout ? pkout : _("authentication failed"));
+        virReportError(VIR_ERR_AUTH_FAILED, "%s", _("authentication failed"));
     }
 
     VIR_FREE(pkout);