netinfo.py: adding 'link_detected' to get_interface_info

The 'link_detected' tells the caller if the network interface
is attached to a network cable (or other media). This information
is only attainable if the network interface is up, thus querying
'link_detected' will return 'n/a' in the interface is down.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>