adding 'official' way to dump SEV VMSA

[Cole Robinson <crobinso@redhat.com>]

Hi all,

SEV-ES and SEV-SNP attestation require a copy of the initial VMSA to
validate the launch measurement. For developers dipping their toe into
SEV-* work, the easiest way to get sample VMSA data for their machine is
to grab it from a running VM.

There's two techniques I've seen for that: patch some printing into
kernel __sev_launch_update_vmsa, or use systemtap like danpb's script
here: https://gitlab.com/berrange/libvirt/-/blob/lgtm/scripts/sev-vmsa.stp

Seems like this could be friendlier though. I'd like to work on this if
others agree.

Some ideas I've seen mentioned in passing:

- debugfs entry in /sys/kernel/debug/kvm/.../vcpuX/
- new KVM ioctl
- something with tracepoints
- some kind of dump in dmesg that doesn't require a patch

Thoughts?

Thanks,
Cole