qmp: Add announce-self command

qmp: Add announce-self command

Add a qmp command that can trigger guest announcements.

It uses its own announce-timer instance, and parameters
passed to it explicitly in the command.

Like most qmp commands, it's in the main thread/bql, so
there's no racing with any outstanding timer.

Based on work of Germano Veit Michel <[email protected]> and
                 Vladislav Yasevich <[email protected]>

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Jason Wang <[email protected]>