Add support for Intel Trust Domain Extensions (TDX)

[jakecorrenti]

This PR adds support for the Intel Trust Domain Extensions (TDX) Confidential Computing architecture.

This is currently a draft as the following issues are present:

• The guest is failing to complete the boot sequence. I suspect this is due to firmware issues, such as a lack of proper IDT setup and #VE handling
• https://www.github.com/virtee/tdx needs to get published to crates.io before this can get merged

Before merging there are some commits that will be squashed and/or re-ordered.

There is also additional functionality that I would like to add such as:

• [ ] Comprehensive CPUID configuration based off of the TDX capabilities reported by KVM_TDX_CAPABILITIES
• [ ] Handle the following VMCALLs
  • [x] TDG.VP.VMCALL<SetupEventNotifyInterrupt>
  • [ ] TDG.VP.VMCALL<GetQuote>
  • [x] TDG.VP.VMCALL<MapGPA>
  • [x] TDG.VP.VMCALL<REPORT_FATAL_ERROR>
• [ ] Validate TDX Attributes when reported by KVM_TDX_CAPABILITIES
• [ ] Update README.md and other docs
• [ ] Make sure guests work with varying memory and vCPU configurations

Any early reviews are welcome.

[jakecorrenti]

Temporarily pushing a mess of commits so that it can get cleaned up...

[jakecorrenti]

closing in favor of https://github.com/containers/libkrun/pull/313