Hi everyone. After a grate help from 'dodo-dk' (You're the best man) and a lot of try and errors i managed to run DSM 6.2.1u6 on Proxmox 5.3-11. I'll share my insights and maybe it'll be useful for someone. Proxmox does have 4 LAN cards model to chose from the list, but as we all know Synology dropped drivers for some devices (e1000, vmware3...) so new version of DSM stopped working on Proxmox. But Proxmox allows you to chose different models (not shown in choosing list) , we just have to add it to 'args:' parameters, just like we did with usb boot key. For this to work you need to know few things:
1) This won't work on OVS bridge/bond - it has to be Linux bridge/bond
2) The Linux bridge/bond has to have gateway set
When we are adding LAN card from 'args:' it looks like the LAN card chooses bridge/bond on which the gateway is set, so that's why it's needed to be set.
First thing you may check it's if e1000e LAN card is available in your kvm. So in Proxmox shell write command:
kvm -device help
Under 'Network devices' you should see line:
name "e1000e", bus PCI, desc "Intel 82574L GbE Controller"
The next step is to find your 'args:' parameters. To do this we need to use another command:
qm showcmd YOUR-VM-ID
YOUR-VM-ID - is ID of your Xpenology machine with added e1000 LAN card. From the output you need to copy two arguments: netdev and device. In my case it looks like this:
For this to work you have to remove the 'script' parts from 'netdev'' and change model from 'e1000' to 'e1000e' in device argument. you can also remove the " ' " signs. After changes it should look like this:
Next you need to add changed arguments in to the VM config file (/etc/pve/qemu-server/YOUR-VM-ID.conf). In my case 'args:' line looks like this:
args: -device ich9-usb-ehci1,id=usb,multifunction=on,bus=pci.0,addr=0xa -drive file=/var/lib/vz/images/103/vm-103-USB103b.raw,format=raw,if=none,id=drive-usb-disk2,cache=writeback -device usb-storage,bus=usb.0,port=2,drive=drive-usb-disk2,id=usb-disk2,bootindex=1,removable=off -netdev type=tap,id=net0,ifname=tap103i0 -device e1000e,mac=BA:E2:8A:3B:AC:3E,netdev=net0,bus=pci.0,addr=0x12,id=net0
The last thing is to remove the 'net' device from config file, so just comment or remove the line with 'net' device. In Proxmox web interface the LAN device wont be visible, but it will work.
So, summing things up, I'm not Proxmox specialist so i didn't knew all of that. One more time thank you 'dodo-dk' for your knowledge and time. God bless all Xpenology enthusiasts, Xpenology devs and Proxmox for being best free virtualization tool i know.