semaphore: fix a hangup problem under load on NetBSD hosts.

semaphore: fix a hangup problem under load on NetBSD hosts.

Fix following bugs in "fallback implementation of counting semaphores
with mutex+condvar" added in c166cb72f1676855816340666c3b618beef4b976:
 - waiting threads are not restarted properly if more than one threads
   are waiting unblock signals in qemu_sem_timedwait()
 - possible missing pthread_cond_signal(3) calls when waiting threads
   are returned by ETIMEDOUT
 - fix an uninitialized variable
The problem is analyzed by and fix is provided by Noriyuki Soda.

Also put additional cleanup suggested by Laszlo Ersek:
 - make QemuSemaphore.count unsigned (it won't be negative)
 - check a return value of in pthread_cond_wait() in qemu_sem_wait()

Signed-off-by: Izumi Tsutsui <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Message-id: 1372841894[email protected]
Signed-off-by: Anthony Liguori <[email protected]>