dns: powerdns: correctly handle different records types (A / AAAA)

dns: powerdns: correctly handle different records types (A / AAAA)

This fixes an issue with dual stacking, when using a zone with both a
IPv4 and IPv6 subnet and the same DNS suffix, pve-network will try to
set both DNS records (type A and AAAA) in the same powerdns rrset,
causing an API error, and effectively causing no forward DNS records
being created.

This change edits the `get_zone_rrset` function so that it takes the
DNS record type into account.

Signed-off-by: Matthieu Pignolet <m@mpgn.dev>
Tested-by: Stefan Hanreich <s.hanreich@proxmox.com>
 [TL: wrap commit message]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>