proxmox-ve: Fix IPv6 configuration in routed setup

[Manawyrm]

This change updates the IPv6 network configuration for Proxmox VE hosts in a routed configuration.
Previously, the proposed configuration would create 3 /64 routes on the system.

• 1 route generated by the vmbr0 interface being a /64
• 1 route created manually with the up ip -6 route add for the same /64
• 1 route created by the eth0 interface being a /64 as well (wrongly so, because none of the /64 hosts will be reachable via NDP on the physical L2 to the router)

This change results in 2 routes:

• a single /128 route to the hosts IPv6 address itself on eth0
• a single /64 route to the vmbr0 interface for the whole subnet

I have read and understood the Contributor's Certificate of Origin available at the end of 
https://raw.githubusercontent.com/hetzneronline/community-content/master/tutorial-template.md
and I hereby certify that I meet the contribution criteria described in it.

Signed-off-by: Sarah Maedel <[email protected]>

[svenja11]

Thank you @Manawyrm!