Reflect missing QEMU/KVM memory usage categories in code

[OhmSpectator]

@cperakis has provided a report on QEMU/KVM memory usage: https://docs.google.com/document/d/1akU0yJTeufqHwy0MfTsd6fO4eZ-ELMk1fJm14YYfblo/edit?usp=sharing.
While the work is ongoing and further experiments are planned to gather more details about memory usage, it has become evident that some categories of memory usage during guest runs are not accounted for in our code. For example, memory used by KVM and some emulation-related allocs are not currently considered.

In our code, we have functions to estimate memory usage for some categories, but they are incomplete. The relevant code snippet is:

eve/pkg/pillar/hypervisor/kvm.go

Lines 522 to 523 in af68942

	overhead = undefinedVMMOverhead() + ramVMMOverhead(domainRAMSize) +
	qemuVMMOverhead() + cpuVMMOverhead(domainMaxCpus, domainVcpus) + mmioOverhead

Even though testing is not yet finished, we can improve this code by listing the categories for which we should calculate memory usage. This ticket aims to enhance the current code to at least include the expected categories, using mock function implementations. The actual implementations will be completed later once all tests are done and analyzed.

So the following can be done:

• Review the current memory usage accounting functions.
• Identify the missing categories of memory usage.
• Add placeholders for the missing categories in the code.
• Implement mock functions for the newly identified categories.
• Document the expected categories and their importance in the code comments.