rpc: socket: Don't repeatedly attempt to launch daemon

On every socket connect(2) attempt we were re-launching session
libvirtd, up to 100 times in 5 seconds.

This understandably caused some weird load races and intermittent
qemu:///session startup failures

https://bugzilla.redhat.com/show_bug.cgi?id=1271183
(cherry picked from commit 2eb7a975)

src/rpc/virnetsocket.c

@@ -547,6 +547,7 @@ int virNetSocketNewConnectUNIX(const char *path,
     virSocketAddr remoteAddr;
     char *rundir = NULL;
     int ret = -1;
+    bool daemonLaunched = false;
 
     VIR_DEBUG("path=%s spawnDaemon=%d binary=%s", path, spawnDaemon,
         NULLSTR(binary));
@@ -627,8 +628,12 @@ int virNetSocketNewConnectUNIX(const char *path,
             goto cleanup;
         }
 
-        if (virNetSocketForkDaemon(binary) < 0)
-            goto cleanup;
+        if (!daemonLaunched) {
+            if (virNetSocketForkDaemon(binary) < 0)
+                goto cleanup;
+
+            daemonLaunched = true;
+        }
 
         usleep(5000);
     }