Reintroduce `name` attribute for (guest) capabilities

Maintain compatibility for vagrant-vbguest plugin which uses it. vagrant-vbguest doesn't use the caps system (to keep compatibility with older Vagrant versions), but includes similar functionality itself.

The `name` attribute we can also use ourself for logging.

The second commit tweaks logging a bit more:
- Use downcase logger name as everywhere else
- Remove duplicate logging from the Guest