Add 'debug-threads' suboption to --name

Add flag storage to qemu-thread-* to store the namethreads flag

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>