Introduce utils.IsValidIP{4,6}()

This patch introduces functions to check for valid IPv4 and IPv6
addresses and converts IsValidIP() to return True if it is either a IPv4
or a IPv6 address.

For now we do not change the functional behavior and replace IsValidIP
with IsValidIP4. This might change in the future.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>