vl.c: Tidy up message printed when we exit on a signal

Tidy up the message printed when qemu exits due to a signal, so that
it's clearer where the message is coming from and that it's not just
stray debug output.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>