format_vl_api_prefix_t returns a dict with keys 'address' and 'address_length',
but other format_vl_api_prefix functions return a dict with 'prefix', and 'len'.
Refactor all format_vl_api_prefix_t to return consistent keys 'address' and 'len'.
Type: refactor
Change-Id: I5f9558fc2da8742a303266e011102f5b2db80aad
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>