vircgroupv2: Fix VM startup when legacy cgroups are defined

On Fedora 31, starting a 'mock' build alters /proc/$pid/cgroup,
probably due to usage of systemd-nspawn.

Before:
$ cat /proc/self/cgroup
0::/user.slice/user-1000.slice/...

After:
$ cat /proc/self/cgroup
1:name=systemd:/
0::/user.slice/user-1000.slice/...

The cgroupv2 code mishandles that first line in the second case, which
causes VM startup to fail with: Unable to read from
'/sys/fs/cgroup/machine/cgroup.controllers': No such file or directory

The kernel docs[1] say that the cgroupv2 path will always start with
'0::', which in the code here controllers="". Only set the v2 placement
path when we see that cgroup file entry.

[1] https://www.kernel.org/doc/html/v5.3/admin-guide/cgroup-v2.html#processes

https://bugzilla.redhat.com/show_bug.cgi?id=1751120Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>

src/util/vircgroupv2.c

@@ -194,12 +194,16 @@ virCgroupV2DetectMounts(virCgroupPtr group,
 static int
 virCgroupV2DetectPlacement(virCgroupPtr group,
                            const char *path,
-                           const char *controllers ATTRIBUTE_UNUSED,
+                           const char *controllers,
                            const char *selfpath)
 {
     if (group->unified.placement)
         return 0;
 
+    /* controllers == "" indicates the cgroupv2 controller path */
+    if (STRNEQ(controllers, ""))
+        return 0;
+
     /*
      * selfpath == "/" + path="" -> "/"
      * selfpath == "/libvirt.service" + path == "" -> "/libvirt.service"