SEV-SNP KVM Support

[AlexOrozco1256]

We are working with the project Oak team to enable running Oak Containers on cloud hypervisor. These containers are SEV-SNP enlightened and can be used alongside with the rest of the oak suite for attestation and measurement of the cvm.

Here is the functionality we are adding to CHV.

1. Implmenting the sev-snp APIs previously defined (sev_snp_init, import_isolated_pages, complete_isolated_import).
2. Expanded the igvm loader to support booting KVM + SEV-SNP enabled VMs. (Note that we can also use the igvm loader to boot VMs with standard cloud images with/without sev-snp enabled as seen here (https://github.com/AlexOrozco1256/chv-demo)
3. Added a fw_cfg device with DMA enabled. Oak's Stage0 firmware uses fw_cfg for loading various parameters into the VM.

Additional design/implementation details can be found here: https://tinyurl.com/chv-kvm-sev-snp
Accompanying pull request: #6993