netinfo: there may be up to one network attached directly to an Iface

Since one nic could be only used for one non-VLANed bridge or one non-VLANed
bridgeless network, it needn't continue to check other networks after
found one.

Change-Id: I80c47d0f0df70de73993ef0c09f69cbf882f6629
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/9737
Reviewed-by: Dan Kenigsberg <danken@redhat.com>