incusd/instance/qemu: Fix handling of > 64 limits.cpu

[stgraber]

Sponsored-by: ActivePort (https://www.activeport.com.au)

[stgraber]

For whoever reviews this, we were getting the issue because on x86_64, CPUs are always hotplugged unless a specific pin is in place. So in the > 64 case without hotplug on x86_64, the cpuCount was set to 1 which would lead to an upper limit of 64 and therefore be immediately hit when hot plugging the remaining CPUs.

With the update, the hotplug case can now expose a separate cpuRequested with the eventual CPU count, this then sets the max to match that and allows things to behave while still not raising the max above 64 except for those cases.