virttest.utils_net: move net related functions to utils_net

Move the net related function to utils_net to ignore the
loop import problems.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.utils_net: Convert windows format ip to linux ip format in update_mac_ip_address

Signed-off-by: Feng Yang <fyang@redhat.com>