Given the impact of libvirt tunnelling mechanism this patch changes
default value of live_migration_tunnelled to False. There are few
reasons to do that:
* Internal libvirt mechanisms increases number of memory copies and
it is just a single-threaded encryption mechanism. Because of that
transfer between source and destination is around 2Gb/s on a 10Gb
network. Given how fast memory is it effectively prevents live
migrations from finishing without entering post-copy mode. Most
operators are turning tunnelling off to increase convergence without
using such hammers like post-copy or pausing an instance during
live migration.
* It also has some limitations, e.g., selective disk migration does
not work when tunnelling is enabled, which means that by default it
is not possible to block live migrate image-backed VMs with additional
volumes atteched
* Default value of live_migration_tunnelled - None - supposed to
choose the best option for encrypting live migration basing on
e.g., availability of native encryption in hypervisor. However,
this will only solve problems with LM limitations when libvirt
tunnelling is in use. Poor performance will still be an issue.
Change-Id: I0323e8bb2ded938522c15b220b84ddfc606e1c97