virsh: Track when create pkttyagent

https://bugzilla.redhat.com/show_bug.cgi?id=1374126

Due to how the processing for authentication using polkit works, the
virshConnect code must first "attempt" an virConnectOpenAuth and then
check for a "special" return error code VIR_ERR_AUTH_UNAVAILABLE in
order to attempt to "retry" the authentication after performing a creation
of a pkttyagent to handle the challenge/response for the client.

However, if pkttyagent creation is not possible for the authentication
being attempted (such as perhaps a "qemu+ssh://someuser@localhost/system"),
then the same failure pattern would be returned and another attempt to
create a pkttyagent would be done. This would continue "forever" until
someone forced quit (e.g. ctrl-c) from virsh as the 'authfail' was not
incremented when creating the pkttyagent.

So add a 'agentCreated' boolean to track if we've attempted to create the
agent at least once and force a failure if that creation returned the same
error pattern.

This resolves a possible never ending loop and will generate an error:

error: failed to connect to the hypervisor
error: authentication unavailable: no polkit agent available to authenticate action 'org.libvirt.unix.manage'

NB: If the authentication was for a sufficiently privileged client, such as
qemu+ssh://root@localhost/system, then the remoteDispatchAuthList "allows"
the authentication to use libvirt since @callerUid would be 0.

tools/virsh.c

@@ -145,6 +145,7 @@ virshConnect(vshControl *ctl, const char *uri, bool readonly)
     bool keepalive_forced = false;
     virPolkitAgentPtr pkagent = NULL;
     int authfail = 0;
+    bool agentCreated = false;
 
     if (ctl->keepalive_interval >= 0) {
         interval = ctl->keepalive_interval;
@@ -166,10 +167,12 @@ virshConnect(vshControl *ctl, const char *uri, bool readonly)
             goto cleanup;
 
         err = virGetLastError();
-        if (err && err->domain == VIR_FROM_POLKIT &&
+        if (!agentCreated &&
+            err && err->domain == VIR_FROM_POLKIT &&
             err->code == VIR_ERR_AUTH_UNAVAILABLE) {
             if (!pkagent && !(pkagent = virPolkitAgentCreate()))
                 goto cleanup;
+            agentCreated = true;
         } else if (err && err->domain == VIR_FROM_POLKIT &&
                    err->code == VIR_ERR_AUTH_FAILED) {
             authfail++;