QMP: Initial support

QMP: Initial support

This commit adds initial QMP support in QEMU. It's important
to notice that most QMP code will be part of the Monitor.

Input will be read by monitor_control_read(). Currently it
does nothing but next patches will add proper input support.

The function monitor_json_emitter(), as its name implies, is
used by the Monitor to emit JSON output. In this commit it's
used by monitor_control_event() to print our greeting message.

Finally, control mode support is also added to monitor_init(),
allowing QMP to be really enabled.

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