[Feature Request] Hot-plug vCPUs

# Feature Request

We'd like to be able to add (and remove) vCPUs from running firecracker microvms. It appears to be possible to do that with KVM. Examples I've seen define how many vCPUs at most a VM might use and then the actual number it will be using at boot. Then you can add vCPUs via `virsh`.

https://www.unixarena.com/2015/12/linux-kvm-how-to-add-remove-vcpu-to-guest-on-fly.html/

This would allow use to add a "burst" feature when CPU usage spikes.

## Describe the desired solution

An API to modify a running microvm's vCPUs count.

This should notify the guest VM of the change:

```
Dec 16 12:48:28 UA-KVM1 kernel: CPU1 has been hot-added
Dec 16 12:48:28 UA-KVM1 kernel: SMP alternatives: switching to SMP code
Dec 16 12:48:57 UA-KVM1 kernel: smpboot: Booting Node 0 Processor 1 APIC 0x1
Dec 16 12:48:57 UA-KVM1 kernel: kvm-clock: cpu 1, msr 0:3ff87041, secondary cpu clock
Dec 16 12:48:57 UA-KVM1 kernel: TSC synchronization [CPU#0 -> CPU#1]:
Dec 16 12:48:57 UA-KVM1 kernel: Measured 906183720569 cycles TSC warp between CPUs, turning off TSC clock.
Dec 16 12:48:57 UA-KVM1 kernel: tsc: Marking TSC unstable due to check_tsc_sync_source failed
Dec 16 12:48:57 UA-KVM1 kernel: KVM setup async PF for cpu 1
Dec 16 12:48:57 UA-KVM1 kernel: kvm-stealtime: cpu 1, msr 3fd0d240
Dec 16 12:48:57 UA-KVM1 kernel: microcode: CPU1 sig=0x206c1, pf=0x1, revision=0x1
Dec 16 12:48:57 UA-KVM1 kernel: Will online and init hotplugged CPU: 1
```

## Describe possible alternatives

We could give every firecracker microvm access to all cores and only use cgroups to limit actual scheduling time. This is not great though as it might create a lot of CPU steal. We prefer to give full cores when possible.

## Checks

- [x] Have you searched the Firecracker Issues database for similar requests?
- [x] Have you read all the existing relevant Firecracker documentation?
- [x] Have you read and understood Firecracker's core tenets?