feat: implement setting mtu for virtio network device

[lucasl0st]

For some scenarios it is required to set the MTU of the proxmox VM network interface, especially when running behind a VPN.
This adds a MTU field to the NetworkDevice struct of the ProxmoxMachine CRD.

See https://git.proxmox.com/?p=qemu-server.git;a=commit;h=61a14cde8d568e552d3deaab2da76b479b8aca7b

Sets the MTU value of a network machine network device:

[pborn-ionos]

We should not default to 1500 here. If a user has different requirements and those are already part of his preconfigured template, we're actively breaking stuff. So i argue if it's not explicitly set in the ProxmoxMachine CR, we should keep existing behaviour and not care about it.

[mcbenjemaa]

https://goplay.tools/snippet/5aCh1GVWyXM

[lucasl0st]

As requested I removed the default MTU and applied the other suggestions, it should have the original behavior now if no MTU is set

[mcbenjemaa]

Sorry for the delay, I will test this and get back to you.

[mcbenjemaa]

have you executed make test?

it's failing in my machine

[lucasl0st]

have you executed make test?

it's failing in my machine

Yes, it is working on my machine (and it is clearly also working in the Github workflow).
Are you using go 1.20? I ran into some issues with 1.21 before when running the tests

[mcbenjemaa]

LGTM

thanks for your contribution