migration: Adjust locking in migration_maybe_pause()

In migration_maybe_pause() QEMU may yield BQL before waiting for a
semaphore.  However it yields the BQL too early, which logically gives it
chance for the main thread to quickly take the BQL and modify the state to
CANCELLING.

To avoid such race condition from happening at all, always update the
migration states within the BQL.  It'll make sure no concurrent
cancellation can ever happen.

With that, IIUC there's chance we can remove the extra parameter in
migration_maybe_pause() to update active state, but that'll be done
separately later.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-9-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

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