python3: Replace keys() + sort() with sorted()

python3: Replace keys() + sort() with sorted()

The keys() method no longer returns a list, so converting the
return value would be necessary before calling sort() on it;
alternatively, we can just call sorted(), which returns a
sorted list.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>