target/i386/monitor: Return QMP error when SEV is not enabled for guest

target/i386/monitor: Return QMP error when SEV is not enabled for guest

If the management layer tries to inject a secret, it gets an empty
response in case the guest doesn't have SEV enabled, or the binary
is built without SEV:

  { "execute": "sev-inject-launch-secret",
    "arguments": { "packet-header": "mypkt", "secret": "mypass", "gpa": 4294959104 }
  }
  {
      "return": {
      }
  }

Make it clearer by returning an error:

  { "execute": "sev-inject-launch-secret",
    "arguments": { "packet-header": "mypkt", "secret": "mypass", "gpa": 4294959104 }
  }
  {
      "error": {
          "class": "GenericError",
          "desc": "SEV not enabled for guest"
      }
  }

Note: we will remove the sev_inject_launch_secret() stub in few commits,
      so we don't bother to add error_setg() there.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211007161716.453984-7-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>