Use nbd-server instead of xnbd-server
xnbd-server is a dead upstream, and so was removed from Debian Bullseyes.
So let's LAVA use nbd-server which is availlable in Debian Bullseyes. The command line change is simple. All internal LAVA names and protocol are kept to "xnbd" for being backward compatible.
Signed-off-by: Corentin LABBE clabbe@baylibre.com