confd: support infering deletion of veth peer

[troglobit]

When adding an interface veth0a we infer the type, and when the user confirms the VETH peer interface, we automatically create/infer that too. So far so good.

However when removing either end of the VETH pair we do not infer the removal of the peer. This leads to a NETCONF warning:

admin@infix-00-00-00:/> configure 
admin@infix-00-00-00:/config/> no interface veth0a
admin@infix-00-00-00:/config/> leave
Error: Invalid leafref value "veth0a" - no target instance "/if:interfaces/if:interface/if:name" with the same value. (Data location "/ietf-interfaces:interfaces/interface[name='veth0b']/infix-interfaces:veth/peer".)
Failed committing candidate to running: Validation failed

Would be great if we could infer the following:

admin@infix-00-00-00:/config/> no interface veth0b