qmp: add SUSPEND_DISK event

qmp: add SUSPEND_DISK event

Emitted when the guest makes a request to enter S4 state.

There are three possible ways of having this event, as described here:

 http://lists.gnu.org/archive/html/qemu-devel/2012-07/msg02307.html

I've decided to add a new event and make it indepedent of SHUTDOWN.
This means that the SHUTDOWN event will eventually follow the
SUSPEND_DISK event.

I've choosen this way because of two reasons:

 1. Having an indepedent event makes it possible to query for its
    existence by using query-events

 2. In the future, we may allow the user to change what QEMU should
    do as a result of the guest entering S4. So it's a good idea to
    keep both events separated

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>