incusd/device/network: Fix Tap interface MTU when in OVN

[trunet]

When creating a virtual-machine with ovn, the tap interface inherits the MTU from ovs-system and therefore doesn't have the MTU.

This doesn't happen on container veth interface, the logic for containers are working fine.

I believe it's a bug, and this PR fixed the issue for me.

Let me know if you need any changes.

Thanks for the awesome work in incus.

[trunet]

If you want more context, my config is like this:

❯ incus network show UPLINK
config:
  bgp.peers.router01.address: [REDACTED]
  bgp.peers.router01.asn: "[REDACTED]"
  bgp.peers.router02.address: [REDACTED]
  bgp.peers.router02.asn: "[REDACTED]"
  dns.nameservers: [REDACTED]
  ipv4.gateway: [REDACTED].9.1/24
  ipv4.ovn.ranges: [REDACTED].50-[REDACTED].254
  ipv4.routes: [REDACTED].12.0/22
  mtu: "9000"
  ovn.ingress_mode: routed
  volatile.last_state.created: "false"
description: ""
name: UPLINK
type: physical
used_by:
- /1.0/networks/ovn-platform
managed: true
status: Created
locations:
- srv-[REDACTED]
- srv-[REDACTED]
- srv-[REDACTED]
project: default
❯ incus network show ovn-platform
config:
  bridge.mtu: "8942"
  ipv4.address: [REDACTED].12.1/24
  ipv4.nat: "false"
  ipv6.address: none
  network: UPLINK
  volatile.network.ipv4.address: [REDACTED].9.50
description: ""
name: ovn-platform
type: ovn
used_by:
- /1.0/instances/u1
- /1.0/instances/u2
managed: true
status: Created
locations:
- srv-[REDACTED]
- srv-[REDACTED]
- srv-[REDACTED]
project: default

before the change:

27: ovs-system: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether ca:a2:3a:67:ee:26 brd ff:ff:ff:ff:ff:ff
43: tap4536cadb: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ovs-system state UP group default qlen 1000
    link/ether 56:a4:3b:18:f7:e9 brd ff:ff:ff:ff:ff:ff

after the change:

27: ovs-system: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 9a:a5:c0:a8:66:09 brd ff:ff:ff:ff:ff:ff
36: tap14bd52f8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8942 qdisc mq master ovs-system state UP group default qlen 1000
    link/ether 5e:f9:09:04:32:3d brd ff:ff:ff:ff:ff:ff

[trunet]

It's done, also rebased. can you review again @stgraber ?

[stgraber]

Thanks!