doc: "hypervisor" -> "Virtual Machine Monitor (VMM)"

[phip1611]

doc: "hypervisor" -> "Virtual Machine Monitor (VMM)"

In literature, especially those with a microkernel background, virtualization stacks are terminologically (and functionally) split into two components: Hypervisor (kernel-space) and Virtual Machine Monitor (VMM) (user-space).

This terminological distinction also makes sense in monolithic kernel space (such as Linux/KVM), as the functionality is always distributed between kernel- and user-space. So, this commit introduces "Virtual Machine Monitor (VMM)" in the docs and replaces most occurrences of "Hypervisor". I think this to clarifies things.

It is unfortunate that "cloud-hypervisor" uses "hypervisor" in its name, although it is just a VMM for KVM (and other Hypervisor backends).

Some in literature also refer to VMMs as device managers.

I didn't change the NixOS options. But one could deprecate microvm.hypervisor in favor of microvm.vmm.