Implement forgotten backend of virInterfaceIsActive()

Implement forgotten backend of virInterfaceIsActive()

Somehow the backend of this function was never implemented in
libvirt's netcf driver, and nobody noticed until now. (The required
netcf function was already in place, so nothing needs to change
there.)
* src/interface/netcf_driver.c: add in the backend function, and point
                                to it from the table of driver functions.