KVM: SVM: Explicitly require DECODEASSISTS to enable SEV support

Add a sanity check on DECODEASSIST being support if SEV is supported, as
KVM cannot read guest private memory and thus relies on the CPU to
provide the instruction byte stream on #NPF for emulation.  The intent of
the check is to document the dependency, it should never fail in practice
as producing hardware that supports SEV but not DECODEASSISTS would be
non-sensical.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>