added VZNetworkBlockDeviceStorageDeviceAttachment

[saracen]

Further to #142, this adds VZNetworkBlockDeviceStorageDeviceAttachment support.

This PR lacks setting a delegate to monitor state changes to the attachment, mostly because I couldn't figure out the best way to integrate this and it would probably lead to a much larger PR. Hopefully that can be done in a follow-up.

I've updated the MacOS application to ease testing. You can serve an existing raw disk within the bundle directory with qemu-nbd:

qemu-nbd -k $(pwd)/source.sock -f raw -x export --cache=none disk.img

And then run:

./macOS -nbd-url "nbd+unix:///export?socket=source.sock"

The -install option also works well with this argument if you serve a disk created with qemu-img.

This is great news as it gives access to qemu's qcow2 format and linked disks functionality.

Relates to #143

[saracen]

@Code-Hex Is the failing CI something I should look into? I couldn't determine whether the cancelled operation was a CI failure, or whether the CI pipeline was externally cancelled.