openvmm command line doc improvements

[jstarks]

Main issue for tracking openvmm CLI --help improvements:

• Get rid of the escaped \< text, which are caused by clap not parsing markdown but rustdoc requiring valid markdown.
• Reduce duplication between related options (e.g., --com1, --com2)
• Add useful error output when a complex option fails to parse (e.g., print a summary of serial options when an invalid --com1 foo option is passed).
• Consider having some way to explicitly get the extended help for serial ports, disks, etc. As we add more backends for these, and as the backend list becomes dynamic, this will be increasingly useful.