x86/sev: Move sev_guest.h into common SEV header

sev_guest.h currently contains various definitions relating to the
format of SNP_GUEST_REQUEST commands to SNP firmware. Currently only the
sev-guest driver makes use of them, but when the KVM side of this is
implemented there's a need to parse the SNP_GUEST_REQUEST header to
determine whether additional information needs to be provided to the
guest. Prepare for this by moving those definitions to a common header
that's shared by host/guest code so that KVM can also make use of them.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240701223148.3798365-3-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>