Migrate to libnbd

[nirs]

libnbd is a high performance and fully featured nbd client library written in C.
https://libguestfs.org/libnbd.3.html

linbd python bindings:
https://gitlab.com/nbdkit/libnbd/-/tree/master/python

The python binding does not have external documentation, you need to use:

python -c "import nbd; help(nbd)"

We maintain our own python library with less features (but everything we use)
and lower performance (but good enough for our usage).

• May improve performance or be worse, depending on the libnbd python binding
• Get rid of 1400 lines of low level hard code in python!
• May need some tweaks in imageio to adapt
• May need some work on libnbd python binding to improve performance
• Need to start with some performance tests to understand what we get