add interface virCgroupGetAppRoot

because libvirt_lxc's cgroup mountpoint is what it shown
in /proc/self/cgroup.

we can get container's cgroup through virCgroupNew("/", &group),
add interface virCgroupGetAppRoot to help container to
get it's cgroup.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>

src/libvirt_private.syms

@@ -85,6 +85,7 @@ virCgroupDenyDeviceMajor;
 virCgroupDenyDevicePath;
 virCgroupForDomain;
 virCgroupForDriver;
+virCgroupGetAppRoot;
 virCgroupForEmulator;
 virCgroupForVcpu;
 virCgroupFree;

src/util/cgroup.c

@@ -965,6 +965,21 @@ int virCgroupForDriver(const char *name ATTRIBUTE_UNUSED,
 }
 #endif
 
+/**
+* virCgroupGetAppRoot:
+*
+* @group: Pointer to returned virCgroupPtr
+*
+* Returns 0 on success
+*/
+int virCgroupGetAppRoot(virCgroupPtr *group)
+{
+#if defined HAVE_MNTENT_H && defined HAVE_GETMNTENT_R
+    return virCgroupNew("/", group);
+#else
+    return -ENXIO;
+#endif
+}
 
 /**
  * virCgroupForDomain:

src/util/cgroup.h

@@ -47,6 +47,8 @@ int virCgroupForDriver(const char *name,
                        int privileged,
                        int create);
 
+int virCgroupGetAppRoot(virCgroupPtr *group);
+
 int virCgroupForDomain(virCgroupPtr driver,
                        const char *name,
                        virCgroupPtr *group,