daemon: Unlink unix socket paths on shutdown

This patch introduces a internal RPC API "virNetServerClose", which
is standalone with "virNetServerFree".  it closes all the socket fds,
and unlinks the unix socket paths, regardless of whether the socket
is still referenced or not.

This is to address regression bug:
https://bugzilla.redhat.com/show_bug.cgi?id=725702

daemon/libvirtd.c

@@ -1558,6 +1558,7 @@ int main(int argc, char **argv) {
 cleanup:
     virNetServerProgramFree(remoteProgram);
     virNetServerProgramFree(qemuProgram);
+    virNetServerClose(srv);
     virNetServerFree(srv);
     if (statuswrite != -1) {
         if (ret != 0) {