KVM: SEV: Add GHCB handling for termination requests

GHCB version 2 adds support for a GHCB-based termination request that
a guest can issue when it reaches an error state and wishes to inform
the hypervisor that it should be terminated. Implement support for that
similarly to GHCB MSR-based termination requests that are already
available to SEV-ES guests via earlier versions of the GHCB protocol.

See 'Termination Request' in the 'Invoking VMGEXIT' section of the GHCB
specification for more details.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240501071048.2208265-4-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>