vyos.interfaces: T1595: add method to query for interface type
vyos.interfaces: T1595: add method to query for interface type

As of now we only could list the available interfaces for a given interface
type. There was no reverse mapping available which told us that interface
eth0.201 is an ethernet interface or vtun0 is openvpn.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x