URI: /plugins/kimchi/interfaces
Methods:
* GET: Retrieve a summarized list of current Interfaces

Retrieves list of host interfaces, which are  not in use by libvirt networks.

Proposal:

URI: /plugins/kimchi/interfaces
Methods:
* GET: Retrieve summarized list of all host Interfaces  
    * Parameters: _inuse:  true | false

if no filter mentioned then API lists all host interfaces

if inuse =“true" then API lists all host interfaces which are in use by libvirt networks

if inuse=“false” then API lists available host interfaces, which are  not in use by libvirt networks.

From UI point of view, current API call should be changed to /plugins/kimchi/interfaces?_inuse='false' to keep the current functionality unaffected.