Fix daemon auto-spawning

Commit 32a9aac switched libvirt to use the XDG base directories
to locate most of its data/config. In particular, the per-user socket
for qemu:///session is now stored in the XDG runtime directory.
This directory is located by looking at the XDG_RUNTIME_DIR environment
variable, with a fallback to ~/.cache/libvirt if this variable is not
set.

When the daemon is autospawned because a client application wants
to use qemu:///session, the daemon is ran in a clean environment
which does not contain XDG_RUNTIME_DIR. It will create its socket
in ~/.cache/libvirt. If the client application has XDG_RUNTIME_DIR
set, it will not look for the socket in the fallback place, and will
fail to connect to the autospawned daemon.

This patch adds XDG_RUNTIME_DIR to the daemon environment before
auto-starting it. I've done this in virNetSocketForkDaemon rather
than in virCommandAddEnvPassCommon as I wasn't sure we want to pass
these variables to other commands libvirt spawns. XDG_CACHE_HOME
and XDG_CONFIG_HOME are also added to the daemon env as it makes use
of those as well.
(cherry picked from commit efe6c80211)

src/rpc/virnetsocket.c

@@ -97,6 +97,9 @@ static int virNetSocketForkDaemon(const char *binary)
                                              NULL);
 
     virCommandAddEnvPassCommon(cmd);
+    virCommandAddEnvPass(cmd, "XDG_CACHE_HOME");
+    virCommandAddEnvPass(cmd, "XDG_CONFIG_HOME");
+    virCommandAddEnvPass(cmd, "XDG_RUNTIME_DIR");
     virCommandClearCaps(cmd);
     virCommandDaemonize(cmd);
     ret = virCommandRun(cmd, NULL);