MINOR: cli/pools: record the list of pool registrations even when merging them

MINOR: cli/pools: record the list of pool registrations even when merging them

By default, create_pool() tries to merge similar pools into one. But when
dealing with certain bugs, it's hard to say which ones were merged together.
We do have the information at registration time, so let's just create a
list of registrations ("pool_registration") attached to each pool, that
will store that information. It can then be consulted on the CLI using
"show pools detailed", where the names, sizes, alignment and flags are
reported.