ovs-lib: Ability to move ip address and routes.

ovs-lib: Ability to move ip address and routes.

The ability to move IP address and routes between two interfaces
is useful when we want to make a physical interface (say eth0)
as a port of OVS bridge (say breth0) with all its IP address and route
information transferred to OVS bridge. An upcoming commit
uses the new ability.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>