reset: allow registering handlers that aren't called by snapshot loading

reset: allow registering handlers that aren't called by snapshot loading

Snapshot loading only expects to call deterministic handlers, not
non-deterministic ones. So introduce a way of registering handlers that
won't be called when reseting for snapshots.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Message-id: 20221025004327.568476-2-Jason@zx2c4.com
[PMM: updated json doc comment with Markus' text; fixed
 checkpatch style nit]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>