register signal handler after initializing SDL.

SDL library initialization mangles signal handlers, so QEMU should
register them after initializing SDL. This was the case before and code
even have a comment about that. Fix it to be so again.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>