x86/kvm: Add KVM-specific VMMCALL handling under SEV-ES

Implement the callbacks to copy the processor state required by KVM to
the GHCB.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
[ jroedel@suse.de: - Split out of a larger patch
                   - Adapt to different callback functions ]
Co-developed-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-64-joro@8bytes.org