Check NQN validity in CLI

[gbregman]

We use NQNs in most of the CLI commands. In case the NQN is too short or too long we would get an error from SPDK. But this error just says "internal error". We can easily check for this in the CLI side and issue a more specific error.

[pcuzner]

@gbregman will you be validating the format of the nqn for the subsystem and hosts?

[gbregman]

@pcuzner , yes, as is done in the SPDK code. The same validation function is used for subsystem and host NQNs. I'm not sure I'll implement the entire validation code but at least obvious things, like string length, etc.