This patch attempts to clean up the mess with qemu-nbd in the Makefile 
in order to get it building and working on Windows.  I checked 
block-nbd, qemu-img, and qemu-nbd on Windows and Linux and everything 
seems to work.  However, there were some defined(QEMU_IMG) that didn't 
include defined(QEMU_NBD) that now are covered by QEMU_TOOL so I'd like 
Laurent to look them over and see if they were intentional.

Basically, there are no longer qemu-nbd-*.o objects.  qemu-img-*.o have 
-DQEMU_TOOL defined and qemu-img/qemu-nbd use identical copies of those 
objects.  A special object is no longer build for nbd.c either.

My longer goal would be to eliminate as many qemu-img-* objects as 
possible but let's take that one step at a time.

Regards,

Anthony Liguori