[BUG] veth links need special handling for add and delete

[Ali-aqrabawi]

veth links need special handling for add and delete as they are create with single command and deleted by single command,

if we create veth10 --> veth20 using the command:

ip -n red link add veth10 type veth peer name veth20

we will have the device leaf set to the peer link, this create circular dependency issue in the yang module.

• if we try to delete veth10 for instance, it will complain that veth20 has leafref to the veth you are trying to delete.

• if we try to delete both veths iproute2-sysrepo will crash due to max recurision depth at add_node_dependencies().

** suggested solution:
is to have veth link as separate list inside links container, and set the device type to string instead of leafref.