This patch adds a struct type in Go that will handle return values
for QAPI's command types.

The return value of a Command is, encouraged to be, QAPI's complex
types or an Array of those.

Every Command has a underlying CommandResult. The EmptyCommandReturn
is for those that don't expect any data e.g: `{ "return": {} }`.

All CommandReturn types implement the CommandResult interface.

Example:
qapi:
    | { 'command': 'query-sev', 'returns': 'SevInfo',
    |   'if': 'TARGET_I386' }

go:
    | type QuerySevCommandReturn struct {
    |     CommandId string     `json:"id,omitempty"`
    |     Result    *SevInfo   `json:"return"`
    |     Error     *QapiError `json:"error,omitempty"`
    | }

usage:
    | // One can use QuerySevCommandReturn directly or
    | // command's interface GetReturnType() instead.
    |
    | input := `{ "return": { "enabled": true, "api-major" : 0,` +
    |                        `"api-minor" : 0, "build-id" : 0,` +
    |                        `"policy" : 0, "state" : "running",` +
    |                        `"handle" : 1 } } `
    |
    | ret := QuerySevCommandReturn{}
    | err := json.Unmarshal([]byte(input), &ret)
    | if ret.Error != nil {
    |     // Handle command failure {"error": { ...}}
    | } else if ret.Result != nil {
    |     // ret.Result.Enable == true
    | }

Signed-off-by: Victor Toso <victortoso@redhat.com>