conf: Simplify slot allocation

conf: Simplify slot allocation

The current algorithm for slot allocation tries to be clever
and avoid looking at buses / slots more than once unless it's
necessary. Unfortunately that makes the code more complex,
and it will cause problem later on in some situations unless
even more complex code is added.

Since the performance gains are going to be pretty modest
anyway, we can just get rid of the extra complexity and use a
completely  straighforward implementation instead.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>