system/runstate: Fix regression, clarify BQL status of exit notifiers

system/runstate: Fix regression, clarify BQL status of exit notifiers

By changing the way the main QEMU event loop is invoked, I inadvertently
changed the BQL status of exit notifiers: some of them implicitly
assumed they would be called with the BQL held; the BQL is however
not held during the exit(status) call in qemu_default_main().

Instead of attempting to ensuring we always call exit() from the BQL -
including any transitive calls - this change adds a BQL lock guard to
qemu_run_exit_notifiers, ensuring the BQL will always be held in the
exit notifiers.

Additionally, the BQL promise is now documented at the
qemu_{add,remove}_exit_notifier() declarations.

Fixes: f5ab12caba4f ("ui & main loop: Redesign of system-specific main
thread event handling")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2771
Reported-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Tested-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>