qemu-thread: implement joinable threads for Win32

Rewrite the handshaking between qemu_thread_create and the
win32_start_routine, so that the thread can be joined without races.
Similar handshaking is done now between qemu_thread_exit and
qemu_thread_join.

This also simplifies how QemuThreads are initialized.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
