virttest.utils_net: Add network and ip address helper classes

1) Add a helper class LibvirtNetwork to create a transient network
   for testing.
2) Add a helper class IPAddress for ease manipulating an IPv4 or IPv6
   address.
3) Add a helper function to retrieve all IP addresses from all
   interfaces.

Signed-off-by: Hao Liu <hliu@redhat.com>