use monitor commands to freeze on pure-v2 setups

use monitor commands to freeze on pure-v2 setups

even an explicit query for the 'unified' cgroup hierarchy
does not work on lxc when run in a pure v2 environment via
the kernel options

    systemd.unified_cgroup_hierarchy=1 cgroup_no_v1=all

so let's use the freeze commands provided by lxc we already
have code for...

See-also: https://forum.proxmox.com/threads/short-read-on-command-socket-error-when-running-backup-of-lxc-containers.82978/

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Tested-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>