fix removing cpulimit on running vm

fix removing cpulimit on running vm

like in pve-container:
04a62bd ("fix #3506: config: fix removing the cpulimit of a running CT")

reported in the forums (no bug# yet):
https://forum.proxmox.com/threads/issue-with-removing-cpu-limit-from-running-vm.97799/

note that this will break CGv1 without the following fix installed:
https://git.proxmox.com/?p=pve-common.git;a=commitdiff;h=d37a71867

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Oguz Bektas <o.bektas@proxmox.com>
Reviewed-by: Oguz Bektas <o.bektas@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>