QMP: Output support

In the new Monitor output is always performed by only two
functions: do_info() and monitor_call_handler().

To support QMP output, we modify those functions to check if we
are in control mode. If so, we call monitor_protocol_emitter()
to emit QMP output, otherwise we do regular output.

QMP has two types of responses to issued commands: success and
error. The outputed data is always a JSON object.

Success responses have the following format:

{ "return": json-value, "id": json-value }

Error responses have the following format:

{ "error": { "class": json-string,
             "desc": json-string,
             "data": json-value } "id": json-value }

Please, note that the "id" key is part of the input code, and
thus is not added in this commit.

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