ovs-lib: Keep internal interface ip during upgrade.

ovs-lib: Keep internal interface ip during upgrade.

Commit 9b5422a98f81("ovs-lib: Try to call exit before killing.")
introduced a problem where internal interfaces are destroyed and
recreated, losing their IP address.

Commit 9aad5a5a96ba("ovs-vswitchd: Preserve datapath ports across
graceful shutdown.") fixed the problem by changing ovs-vswitchd
to preserve the ports on `ovs-appctl exit`.  Unfortunately, this fix is
not enough during upgrade from <= 2.5.0, where an old ovs-vswitchd is
running (without the fix) and a new ovs-lib script is performing the
restart.

The problem seem to affect both RHEL and ubuntu.

This commit fixes the upgrade by looking at the running daemon
version and avoid using `ovs-appctl exit` if it's < 2.5.90.

Suggested-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>