Using the qemu+ssh migration scheme requires tunneling through libvirt
to be enabled, since qemu does not natively support SSH based
migrations.
Since our default configuration uses a secure migration channel, we
should also configure libvirt tunneled migrations by default.
Change-Id: Ia389e78994da0f5f1cd99edeafeef13009906621
(cherry picked from commit cb89b3bee5)