LXC: fix memory leak in lxcContainerMountFSBlockHelper

we alloc the memory for format in lxcContainerMountDetectFilesystem
but without free it in lxcContainerMountFSBlockHelper.

this patch just call VIR_FREE to free it.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>

src/lxc/lxc_container.c

@@ -940,6 +940,7 @@ static int lxcContainerMountFSBlockHelper(virDomainFSDefPtr fs,
     }
 
 cleanup:
+    VIR_FREE(format);
     return ret;
 }