bgp: T5306: fix verify_remote_as() to support v6only interface with peer-group

To test:

set protocols bgp neighbor eth0 interface v6only peer-group 'fabric'
set protocols bgp peer-group fabric address-family ipv4-unicast
set protocols bgp peer-group fabric address-family ipv6-unicast
set protocols bgp peer-group fabric capability extended-nexthop
set protocols bgp peer-group fabric remote-as 'external'
set protocols bgp system-as 64496