iohandler.c: Properly initialize sigaction struct

The code in qemu_init_child_watch() wasn't clearing the 'struct
sigaction' before passing it to sigaction(); this meant that we
would block a random set of signals while executing the SIGCHLD
handler. Initialize properly by using memset() on the struct,
as we do in similar cases elsewhere.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
