When running on Xen, with interfaces provided by the xen-netback driver, `sg` offload must be enabled before the MTU can be increased beyond 1500.
Currently VyOS appears to be setting MTU first (which fails), then enabling sg.
Discussed in: [forum](https://forum.vyos.io/t/cannot-change-mtu-oserror-errno-22-invalid-argument/5513/7?u=maznu)
Relevant [line 367 of xen-netback/interface.c](https://elixir.bootlin.com/linux/latest/source/drivers/net/xen-netback/interface.c#L367)
Should just need some rearranging of the code bringing up an interface to ensure MTU happens after SG.