main_loop: Make main_loop_wait() return void

main_loop: Make main_loop_wait() return void

The last users of main_loop_wait() that cared about the return value
have now been changed to no longer use it. Drop the now-useless return
value and make the function return void.

We avoid the awkwardness of ifdeffery to handle the 'ret'
variable in main_loop_wait() only being wanted if CONFIG_SLIRP
by simply dropping all the ifdefs. There are stub implementations
of slirp_pollfds_poll() and slirp_pollfds_fill() already in
stubs/slirp.c which do nothing, as required.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1498584769-12439-3-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>