libvirt: lxc: fix incorrect parameter of lxcContainerMountProcFuse

when we has no host's src mapped to container.
there is no .oldroot dir,so libvirt lxc will fail
to start when mouting meminfo.

in this case,the parameter srcprefix of function
lxcContainerMountProcFuse should be NULL.and make
this method handle NULL correctly.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>

src/lxc/lxc_container.c

@@ -605,7 +605,7 @@ static int lxcContainerMountProcFuse(virDomainDefPtr def,
 
     if ((ret = virAsprintf(&meminfo_path,
                            "%s/%s/%s/meminfo",
-                           srcprefix, LXC_STATE_DIR,
+                           srcprefix ? srcprefix : "", LXC_STATE_DIR,
                            def->name)) < 0)
         return ret;
 
@@ -2059,7 +2059,7 @@ static int lxcContainerSetupExtraMounts(virDomainDefPtr vmDef,
         goto cleanup;
 
     /* Mounts /proc/meminfo etc sysinfo */
-    if (lxcContainerMountProcFuse(vmDef, "/.oldroot") < 0)
+    if (lxcContainerMountProcFuse(vmDef, NULL) < 0)
         goto cleanup;
 
     /* Now we can re-mount the cgroups controllers in the