Remove sub-mounts under /dev when starting an LXC container

Since we are mounting a new /dev in the container, we must
remove any sub-mounts like /dev/shm, /dev/mqueue, etc,
otherwise they'll be recorded in /proc/mounts, but not be
accessible to applications.

src/lxc/lxc_container.c

@@ -1416,10 +1416,11 @@ static int lxcContainerSetupPivotRoot(virDomainDefPtr vmDef,
     if (lxcContainerPivotRoot(root) < 0)
         goto cleanup;
 
-    /* Gets rid of any existing stuff under /proc, since we need new
-     * namespace aware versions of those. We must do /proc second
-     * otherwise we won't find /proc/mounts :-) */
+    /* Gets rid of any existing stuff under /proc, /sys & /tmp
+     * We need new namespace aware versions of those. We must
+     * do /proc last otherwise we won't find /proc/mounts :-) */
     if (lxcContainerUnmountSubtree("/sys", false) < 0 ||
+        lxcContainerUnmountSubtree("/dev", false) < 0 ||
         lxcContainerUnmountSubtree("/proc", false) < 0)
         goto cleanup;