qemu-thread: add a fast path to the Win32 QemuEvent

QemuEvents are used heavily by call_rcu.  We do not want them to be slow,
but the current implementation does a kernel call on every invocation
of qemu_event_* and won't cut it.

So, wrap a Win32 manual-reset event with a fast userspace path.  The
states and transitions are the same as for the futex and mutex/condvar
implementations, but the slow path is different of course.  The idea
is to reset the Win32 event lazily, as part of a test-reset-test-wait
sequence.  Such a sequence is, indeed, how QemuEvents are used by
RCU and other subsystems!

The patch includes a formal model of the algorithm.

Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>