rbd: use pve-storage-portal-dns-list for monhost

rbd: use pve-storage-portal-dns-list for monhost

This way we get parameter verification on monitor addresses
as well as the ability to pass multiple `--monhost`
arguments to `pvesm add`.

Since our '-list' schemas default to using commas we now
need to properly support these, so all uses of the monhost
property now replace all of semicolon, space or comma into
the currently required character.
This should fix the issues reported by Alwin Antreich on the
pve-user list.

Since this schema supports both ipv6+port notations we need
to make sure we convert to the bracket enclosed variant.
Added a helper for this.