This patch from Laurent Vivier (with minor changes by Paul Clements) 
adds partition support to NBD. By default this is turned off, but can be 
enabled with a new module parameter, max_part.

Below, Laurent notes that partition table re-read doesn't automatically 
occur. For now you'll have to open("/dev/nbdX") to get that to happen.
Longer term, nbd-client can be modified to fork just before nbd_do_it() 
and then call ioctl(BLKRRPART, "/dev/nbdX") in the parent process 
(probably need to wait for the "pid" file to show up in /sys to avoid a 
race).

Thanks,
Paul