lxd/device/nic_ovn: Prevent setting static IPv6 if static IPv4 is not set

[MusicDin]

Due to a bug in OVN, we should not allow setting static IPv6 if static IPv4 is not set.

Closes #12297

[xlmnxp]

What about IPv6-only networks?

[MusicDin]

You can create an IPv6 only OVN network if you disable ipv4.dhcp.

lxc network set ovn ipv4.dhcp=false ipv6.address=fd42:1000:1000:1000::1/64

lxc launch ubuntu:22.04 c1 --network ovn

lxc ls
+------+---------+------+-----------------------------------------------+-----------+-----------+
| NAME |  STATE  | IPV4 |                     IPV6                      |   TYPE    | SNAPSHOTS |
+------+---------+------+-----------------------------------------------+-----------+-----------+
| c1   | RUNNING |      | fd42:1000:1000:1000:216:3eff:feca:818f (eth0) | CONTAINER | 0         |
+------+---------+------+-----------------------------------------------+-----------+-----------+