Return the actual number of bytes written to through command buffer

The `Write` function returned the number of bytes written to the
internal command response buffer in the simulator. Depending on the
type of command executed against the TPM, this would usually result
in a different number of bytes being written than the original input.

The fix is useful if one wants to wrap the io.ReadWriterCloser
functions with implementations that perform strict checks. An example
is using an io.MultiWriter to capture the bytes sent to the TPM for
debugging purposes, which will fail if the number of bytes written
does not equal the original length of the input bytes.

simulator/simulator.go

@@ -109,7 +109,14 @@ func (s *Simulator) Write(commandBuffer []byte) (int, error) {
 	if err != nil {
 		return 0, err
 	}
-	return s.buf.Write(resp)
+	n, err := s.buf.Write(resp)
+	if err != nil {
+		return 0, err
+	}
+	if n != len(resp) {
+		return 0, fmt.Errorf("expected %d bytes to be written to command response buffer, but actual number is %d", n, len(resp))
+	}
+	return len(commandBuffer), nil
 }
 
 // Read gets the response of a command previously issued by calling Write().