Split from Jan's original qemu-thread-posix.c patch.  No semantic change,
just introduce the new API that POSIX and Win32 implementations will
conform to.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>