Support custom linux kernel in Containerplat UVM

[DomAyre]

Current implementation is in https://github.com/microsoft/confidential-aci-testing/tree/custom-vmgs

What it does:

• User generates their own VMGS file from a kernel bzimage using:
  • https://github.com/microsoft/igvm-tooling for generating a kernel.bin file
  • vmgstool binary (Needs sourcing from magical place) to create a VMGS file
• User calls c-aci-testing vm deploy --vmgs-file <path_to_file>
  • This fetches containerplat as normal, but then replaces LinuxBootFiles/kernelinitrd.vmgs with the users .vmgs file
  • Deployment continues as normal
• For the bzimage I was given to test, pod creation doesn't work, not sure if I'm missing something, possible other files need changing too

Todos:

• Make it work (missing files?)
• Automate generating VMGS file from bzimage (Requires standardised place to get vmgstool)