Use nbd.connect_systemd_socket_activation to run nbdkit/qemu-nbd commands

Currently using subprocess.* functions to run qemu-nbd. I think this can be simplified by using libnbd's builtin
connect_systemd_socket_activation and nbd.connect_command. Example:

https://github.com/libguestfs/libnbd/blob/master/python/examples/checksum.py#L35

and:

https://github.com/libguestfs/libnbd/blob/master/python/t/590-aio-copy.py#L122

```
09:51 abi: it's not connected with systemd except that they invented the protocol
09:51 it doesn't require systemd to be installed or running
```

Unfortunately there is currently no way to catch stderr, means if underlying command fails for some reason, errors cannot be
catched and written to logfile accordingly. Need to check if its possible to redirect errors for all commands to a logfile.