CP-50934: fix qemu cgroups to be compatible with cgroupv2 (xapi-project#6330)

cgroup-v2 doesn't have a 'tasks' file anymore (which refers to threads,
not processes). Use cgroup.procs instead which exists on both cgroupv1
and cgroupv2, and would be actually the correct one to use, because we
want to move the entire process (even if it has, or will have multiple
threads).

Tested on cgroup-v1 (XS8):
```
systemd-cgls /sys/fs/cgroup/cpu/vm.slice|grep qemu-dm
└─2421656 qemu-dm-22 -machine pc-i440fx-2.10,accel=xen,max-ram-below-4g=40265...
```

And on cgroup-v2 (XS9):
```
systemd-cgls /sys/fs/cgroup/vm.slice|grep qemu-dm
└─777450 qemu-dm-511 -machine pc-i440fx-2.10,accel=xen,max-ram-below-4g=4026531…
```

Previously in cgroup-v2 qemu has ended up underneath forkexecd, which is
part of control.slice, reducing the amount of CPU available to
oxenstored and XAPI, leading to timeouts on busy machines.

Author: edwintorok

ocaml/xenopsd/scripts/qemu-wrapper

@@ -95,13 +95,18 @@ def prepare_exec():
         # Move to nominated cgroup slice
         print("Moving to cgroup slice '%s'" % cgroup_slice)
         try:
-            # Note the default slice uses /sys/fs/cgroup/cpu/tasks but
-            # other.slice uses /sys/fs/cgroup/cpu/other.slice/tasks.
-            g = open("/sys/fs/cgroup/cpu/%s/tasks" % cgroup_slice, 'w')
+            try:
+                # cgroup-v1 path:
+                # Note the default slice uses /sys/fs/cgroup/cpu/cgroup.procs but
+                # other.slice uses /sys/fs/cgroup/cpu/other.slice/cgroup.procs.
+                g = open("/sys/fs/cgroup/cpu/%s/cgroup.procs" % cgroup_slice, 'w')
+            except FileNotFoundError:
+                # cgroup-v2 path:
+                g = open("/sys/fs/cgroup/%s/cgroup.procs" % cgroup_slice, 'w')
             g.write(str(os.getpid()))
             g.close()
         except IOError as e:
-            print("Warning: writing pid to '%s' tasks file: %s" \
+            print("Warning: writing pid to '%s' cgroup.procs file: %s" \
                 % (cgroup_slice, e))
 
     core_dump_limit = enable_core_dumps()