Introduce yank feature

Introduce yank feature

The yank feature allows to recover from hanging qemu by "yanking"
at various parts. Other qemu systems can register themselves and
multiple yank functions. Then all yank functions for selected
instances can be called by the 'yank' out-of-band qmp command.
Available instances can be queried by a 'query-yank' oob command.

Signed-off-by: Lukas Straub <[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Message-Id: <69934ceacfd33a7dfe53db145ecc630ad39ee47c.1609167865[email protected]>
Acked-by: Marc-André Lureau <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>