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: NGao feng <gaofeng@cn.fujitsu.com>
(cherry picked from commit 73e2d646)

src/lxc/lxc_container.c

@@ -971,6 +971,7 @@ static int lxcContainerMountFSBlockHelper(virDomainFSDefPtr fs,
     }
 
 cleanup:
+    VIR_FREE(format);
     return ret;
 }