Update the counting semaphore implementation per comments on qemu-devel.

http://lists.nongnu.org/archive/html/qemu-devel/2013-07/msg00030.html
 - make QemuSemaphore.count unsigned (it won't be negative)
 - also eliminate checks for negative count values
 - check a return value of in pthread_cond_wait() in qemu_sem_wait()