Hello,

I have an issue whenever using `/proc/sys/ipv6/conf` as read-only mountpoint.

While the libvirt by itself is running fine, I'm unable to create NAT network 
interfaces because of the following error:
```
2025-07-05 19:33:32.821+0000: 100: error : networkSetIPv6Sysctl:1847 : cannot 
write to '/proc/sys/net/ipv6/conf/virbr0/disable_ipv6' on bridge 'virbr0': 
Read-only file system
2025-07-05 19:33:32.841+0000: 100: error : virNetDevSetIFFlag:601 : Cannot get 
interface flags on 'virbr0': No such device
2025-07-05 19:33:32.947+0000: 100: error : virNetlinkDelLink:688 : error 
destroying network device virbr0: No such device
```

While using iproute2 and bridge utils, I'm able to create and configure new 
bridge device, along with attaching the VM to the interface.
I've used the following commands:
```
sysctl -w net.ipv4.ip_forward=1

brctl addbr br0
ip link set br0 up
ip addr add 192.168.100.1/24 dev br0

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i br0 -o eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o br0 -m state --state RELATED,ESTABLISHED -j 
ACCEPT
```

Is libvirt allowing to disable sysctl calls in the configuration files? If so, 
what should I set in the config?

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