Revert "main-loop.c: Handle SIGINT, SIGHUP and SIGTERM synchronously"

This reverts commit 15124e14. It breaks
debuggability of qemu and is no longer needed as the problem has
now been addressed in a different way.

Instead we provide a comment about why these signals must be
handled asynchronously.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
[PMM: added comment]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>