cpu hotplug: cannot change feature online

cpu hotplug: cannot change feature online

The vCPUs are passed as devices with specific id only when CPU
hot-plug is enable at cold start.

So, we can't enable/disable allow-hotplug online as then vCPU hotplug
API will thrown errors not finding core id.

Not enforcing this could also lead to migration failure, as the QEMU
command line for the target VM could be made different than the one it
was actually running with, causing a crash of the target as Fiona
observed [0].

[0]: https://lists.proxmox.com/pipermail/pve-devel/2023-October/059434.html

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
 [ TL: Reflowed & expanded commit message ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>