Move performance counter storage for global variable to virtual machine(s)

[Kouteki]

Regarding the storage of performance counters, they should be stored per virtual machine, instead of a global variable measure, which is generally a bad practice, bringing a lot of unstated constraints and risks. I understand that it is already the case for other parts of the VM, and that it facilitates the coding, but this is something that we should eventually fix. It can be done later, I don't want to block the PR for that.

Originally posted by @mvertes in #2241 (review)

Proposed upgrade of #2241