There are some magic words in _add_option method of qemu_vm module, I hope you could take care of them in this set_param method as well, at least provide the same way to add some particular options in qemu cli.
```
 432             elif value and isinstance(value, str):
 433                 # "EMPTY_STRING" and "NULL_STRING" is used for testing illegal
 434                 # foramt of option.
 435                 # "EMPTY_STRING": set option as a empty string "".
 436                 # "NO_EQUAL_STRING": set option as a option string only,
 437                 #                    even without "=".
 438                 #       (In most case, qemu-kvm should recognize it as "<null>")
 439                 if value == "NO_EQUAL_STRING":
 440                     return ",%s" % option
 441                 if value == "EMPTY_STRING":
 442                     value = '""'
```