spapr watchdog should honour watchdog-set-action etc monitor commands

We have a mechanism for the user to select what they would like to have happen when a watchdog times out -- there are monitor commands like watchdog-set-action so they can select reset, power-off, do-nothing, etc. This is done by the watchdog device calling watchdog_perform_action() to get the user-selected behaviour. We've done this for every non-deprecated watchdog device we have, with one exception: hw/watchdog/spapr_watchdog.c.

This one is trickier, because the device is implementing a watchdog where the guest gets to select what the action to take on timeout should be. We had a mailing list discussion about it here: https://lore.kernel.org/qemu-devel/CAFEAcA_KjSgt-oC=d2m6WAdqoRsUcs1W_ji7Ng2fgVjxAWLZEw@mail.gmail.com/ where we agreed that the behaviour we want is:

• if the user has not said anything about what they want the watchdog action to be: do what the guest asks for
• if the user has asked for a specific watchdog action: do that, and ignore the guest's opinion
• the user should be able to choose "do what the guest asks to do"

This will require expanding the monitor protocol, and probably also tweaking the watchdog_perform_action() etc API.