Add container_uuid env variable to LXC guests

Systemd has declared that all container virtualization technologies
should set 'container_uuid' to identify themselves.

http://cgit.freedesktop.org/systemd/systemd/commit/?id=09b967eaa51a39dabb7f238927f67bd682466dbc

src/lxc/lxc_container.c

@@ -125,6 +125,7 @@ static virCommandPtr lxcContainerBuildInitCmd(virDomainDefPtr vmDef)
     virCommandAddEnvString(cmd, "PATH=/bin:/sbin");
     virCommandAddEnvString(cmd, "TERM=linux");
     virCommandAddEnvString(cmd, "container=lxc-libvirt");
+    virCommandAddEnvPair(cmd, "container_uuid", uuidstr);
     virCommandAddEnvPair(cmd, "LIBVIRT_LXC_UUID", uuidstr);
     virCommandAddEnvPair(cmd, "LIBVIRT_LXC_NAME", vmDef->name);
     if (vmDef->os.cmdline)