Attempt to update to the new Mirage / DNS APIs

[talex5]

However, this doesn't work. We use Resolver.create stack ~nameserver:(`UDP, (nameserver_ip, 53)) to create the DNS resolver, but the logs show:

2020-11-25 16:06:51 -00:00: DBG [ipv4] ip write: mtu is 1500, hdr_len is 20, size 28 payload len 0, needed_bytes 48
2020-11-25 16:06:51 -00:00: DBG [pcb] process-reset: [channels=0 listens=0 connects=1]
2020-11-25 16:06:51 -00:00: DBG [pcb] Refused connection to 10.139.1.1:53
2020-11-25 16:06:51 -00:00: ERR [dns_client_mirage] error connecting to nameserver connection attempt was refused

Which looks a lot like it ignored us and tried to use TCP instead.

A comment here points in the same direction: https://github.com/mirage/qubes-mirage-firewall/blob/cfe122592df6c6bdab7c5d322e7b26a52e77be4e/test/unikernel.ml#L348

/cc @hannesm

[hannesm]

yes, the MirageOS DNS client does not support UDP at the moment (there is special handling code in qubes-firewall since that IP stack is used as both host stack and router stack), see mirage/ocaml-dns#243 -- I have plans to get this done before end of this year.