Add support for multiple vCPUs

[Ivan-Velickovic]

Re-based version of https://github.com/au-ts/libvmm/pull/116.

It should be noted that multi-core virtual machines won't work yet because Microkit lacks support for it, but when Microkit does add support for pinning a virtual CPU to a particular core, there won't be any changes necessary to libvmm to enable multi-core guests.

Remaining TODOs:

• [ ] Update manual
• [ ] Update vGICv3 code for multiple vCPUs.
• [ ] Add a test for multiple vCPUs.

Closes https://github.com/au-ts/libvmm/issues/4.