nbd: Add small python nbd client for future usage in scripts/imgedit.py.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: Build images as QCOW2 files.

Make use of the python nbd client.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>