[Qemu-block] Python NBD client library

[Nir Soffer]

Thanks everyone for the good input on

http://lists.nongnu.org/archive/html/qemu-block/2018-07/msg00168.html

I'm convinced that the best direction for oVirt is having NBD client instead

of using the kernel nbd client. I think a C library would be best, but a python

version is faster to implement and may be good enough for oVirt, so I'm

starting in this direction.

If you like to check the prototype, see:

https://gerrit.ovirt.org/c/93384/

qemu's nbd/client.c was very helpful so far, since NBD doc

https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md

is not very clear.

Not sure that imageio is the best place for this. Do you have a better idea

about a better home for this library?

Here is what we can do now:

    $ qemu-img create -f qcow2 test.qcow2 1g

    Formatting 'test.qcow2', fmt=qcow2 size=1073741824 cluster_size=65536 lazy_refcounts=off refcount_bits=16

    

    $ qemu-nbd --fork -k /tmp/nbd.sock -t -f qcow2 test.qcow2 -x export -n

    

    $ python3

    ...

    >>> from ovirt_imageio_common import nbd

    >>> c = nbd.Client("/tmp/nbd.sock", "export")

    >>> c.export_size

    1073741824

    >>> c.minimum_block_size

    1

    >>> c.preferred_bloc_ksize

    4096

    >>> c.maximum_bloc_ksize

    33554432

Nir