Description of problem:

Plugins don't communicate whether they can actually be used to client code.

Version-Release number of selected component (if applicable):

1.fc21

How reproducible:

Quite.

Steps to Reproduce:

N/A

Actual results:

N/A

Expected results:

N/A

Additional info:

The plugin should have some interface such that client code can invoke
a function, "available", that returns true if the facilities
on which the plugin tool is dependent are available, otherwise false.
They should also have a method "implemented", which distinguishes between
a fully implemented plugin and a dummy plugin, with stub methods. Names "available" and "implemented" have been chosen to match the existing names
in the tasks framework.

libblockdev has facilities for loading modules. There should be some attention
paid to whether the underlying facilities are available when loading a module.
One possibility is that an exception should be raised when the module is being
loaded if its facilities are unavailable. This is probably the most general and best since the client code can then determine the appropriate action, e.g., unload, warn user, etc.