KVM: SVM: Inject #UD on attempted emulation for SEV guest w/o insn buffer

Inject #UD if KVM attempts emulation for an SEV guests without an insn
buffer and instruction decoding is required.  The previous behavior of
allowing emulation if there is no insn buffer is undesirable as doing so
means KVM is reading guest private memory and thus decoding cyphertext,
i.e. is emulating garbage.  The check was previously necessary as the
emulation type was not provided, i.e. SVM needed to allow emulation to
handle completion of emulation after exiting to userspace to handle I/O.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>