Make qemu_shutdown_requested signal-safe

qemu_shutdown_requested may be interrupted by qemu_system_killed. If the
latter sets shutdown_requested after qemu_shutdown_requested has read it
but before it was cleared, the shutdown event is lost. Fix this by using
atomic_xchg.

This provides a different fix for the problem which commit 15124e142
attempts to deal with. That commit breaks use of ^C to drop into gdb,
and so this approach is better (and 15124e142 can be reverted).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[PMM: commit message tweak]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Revert "main-loop.c: Handle SIGINT, SIGHUP and SIGTERM synchronously"

This reverts commit 15124e142034d21341ec9f1a304a1dc5a6c25681. 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>