Fix semaphores fallback code

As reported in bug 1087114 the semaphores fallback code is broken which
results in QEMU crashing and making QEMU unusable.

This patch is from Paolo.

This needs to be back ported to the 1.3 stable tree as well.

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