Rework command line interface

[lorc]

Current xu command line interface is clunky. For example, for some reason we use flags for mandatory options, which is defiles established practices. Moreover flags are locked in positions. For example #57 introduces new -c flag for xu create command, but you can't use change order of flags. Only this command is possible:

# xu create -d <domid> -c <config_name>

You can't even swap the arguments:

# xu create -c <config_name> -d <domid>  # this will not work

I propose to align use of flags with established standard: only optional parameters needs flag, all mandatory options are positional and require no additional flags. For example:

# xu console 1
# xu create domd -d 1
# xu create domd
# xu destroy 1

This is a great time for this, before we release the fist "stable" version.

[GrygiriiS]

Before reworking cmd interface it shell be documented. Or documentation shell be created for the new one.