"kdd" Windows debugger stub.

"kdd" Windows debugger stub.

kdd runs in dom0, attaches to a domain and speaks the Windows kd serial
protocol over a TCP connection (which should go to kd or windbg, e.g.
by having another VM with its virtual COM1 set up as a TCP listener).

It doesn't do breakpoints &c yet, and windbg can get quite confused
since the kernel debugger's not actually running, but it's good enough
to extract backtraces from wedged VMs.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>