Move the unit context to a name matching the other contexts. Remove some
unnecessary entries from it that are unused, and remove references to
the old name relying on spec helper to load all contexts.
Ensure that the tests always set VAGRANT_HOME into the environment to
prevent the global plugin manager from trying to load the plugins under
this directory.
Vagrant newer than 2.2.11 reworked how the after hook definition
functions requiring it to be called in a different way to ensure it is
possible to hook the box remove action to perform the expected local
action.
Add a compatibility function to ensure that the plugin works with both
mechanisms and some simple tests to ensure that with the unit tests
validated against the older vagrant versions that it confirms the action
will be called as expected.
As part of fixing the call of the remove_libvirt_image action, ensure it
only executes when the box removed is a libvirt one and ignores all
others.
Fixes: #1196
