MINOR: pools: support creating a pool from a pool registration

MINOR: pools: support creating a pool from a pool registration

We've recently introduced pool registrations to be able to enumerate
all pool creation requests with their respective parameters, but till
now they were only used for debugging ("show pools detailed"). Let's
go a step further and split create_pool() in two:
  - the first half only allocates and sets the pool registration
  - the second half creates the pool from the registration

This is what this patch does. This now opens the ability to pre-create
registrations and create pools directly from there.