main loop: use msec-based timeout in glib_select_fill

The timeval-based timeout is not needed until we actually invoke select,
so compute it only then.  Also group the two calls that modify the
timeout, glib_select_fill and os_host_main_loop_wait.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
