Fix live migration under xl

Until now the only way to make live migration work in conjunction with
"xl" was to add ssh known_hosts keys for every node's secondary ip on
every other node.

With this command we remove the target key verification: this is not
worse than what we were doing before with "xm", and allows the migration
to happen under either toolstack, without extra manual work. Of course
the full security of ssh is not used by live migration, then.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>