Fix windows build after init_host_timer changes.

host_alarm_timer fires in a separate thread.  The windows build current
uses SetEvent() and WaitEvent() to then notify the main thread.  This is
functionally equivalent to what we're doing in Unix with pipe().  So let's
just #ifdef the pipe() code on Windows since it doesn't build there anyway.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5637 c046a42c-6fe2-441c-8c8c-71466251a162