fix win32 build

On Windows, cpus.c needs access to the hThread.  Add a Windows-specific
function to grab it.  This requires changing the CPU threads to
joinable.  There is no substantial change because the threads run
in an infinite loop.

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