gnt-instance info: remove hvattr descriptions

Having hvattr descriptions is only confusing for the user, because even
if they explain better what an attribute is about, they don't help in
deciding what keyword should be used to actually set it. If in the
future we want descriptions they should probably live in constants.py,
and be displayed together with the key, rather than instead of it.

This patch also changes the handling of the vnc console connection
description, making compatible work with both kvm and xen-hvm.

Reviewed-by: iustinp