QMP: Return an empty dict by default

Currently, when a regular command doesn't have any data to output,
QMP will emit:

{ "return": "OK" }

Returning an empty dict is better though, because dicts can support
some protocol changes in a compatible way.

So, with this commit we will return:

{ "return": {} }

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>