torusblk: support nbd detach command

[nak3]

No description provided.

[barakmich]

Perhaps you could help explain the desired lifecycle? Right now, as long as the process is running, it is attached, and it detaches on exit.

[nak3]

Sure. This patch solves two issues:

1. When we run torusblk nbd <VOLUME> /dev/nbdN as background process, we need to stop the process with kill command.
2. If torusblk nbd process failed to clean up device during its exit, there are no way to clean up the device by users.

This patch still keep the life cycle, attached during running process and detached during the exit. I am thinking that administrators need this command for above two reason.

[barakmich]

I see. would it be possible to make "attach" the default action for torusblk nbd?

[nak3]

Sure, I updated. One reason why I wanted to add attach subcommand is that, torusblk nbd's help output conflict with --detach option.

It says that the command need VOLUME, but --detach option needs only NBD-DEV.

Usage:
  torusblk nbd VOLUME [NBD-DEV] [flags]

So, I added the example in Flag output.

Flags:
  -d, --detach string   detach an NBD device from a block volume. (e.g. torsublk nbd -d /dev/nbd0)

[barakmich]

Good compromise! LGTM