netinfo: get an interface using one of IP addresses assigned to it

We record the server's IP address for each XMLRPC request, preventing
manually parsing the routes files.
This patch also fix an error that when connected from localhost(using
tools like vdsClient) through the loopback interface, vdsm would report
the interface associated with the default gateway as the used interface,
instead of the expected result 'lo', because the routing table doesn't
contain any information for the device 'lo'.

Change-Id: I239f92c527524e5651f51712841c3eafcf659e53
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/11519
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>