Deal with drivers priority

This is the first attempt of dealing with drivers priority. The counter part commit, on osinfo-db, can be found here: osinfo-db!44 (merged)

This API is horrible as mgmt apps would have to:

• Get the list of prioritized drivers;
• Get the priority of the first driver;
• Loop in the drivers while their priority is the same as the priority of the first drivers;

However, I don't see a better way deal with this right now if we want to deal with fallback drivers.

@berrange, @crobinso, @felipeborges, do you guys have some ideas?

One thing that can be done is just:

• get_prioritized_drivers() returning the ones with the highest priority;
• get_fallback_drivers() returning the second ones;

But, by doing this, we're limiting mgmt apps to have only one fallback mechanism.

Anyways, suggestions are more than welcome.