Update migration defaults

In TripleO we wanted to enable live block migration. Pleasantly
surprising was finding that all it required was better options to
libvirt.

I'm told this works with the libvirt we gate on, though as we don't
gate migration yet, its hard to tell.

Change-Id: I7b997666474ec324c5034e3660a7271b8423bca8
DocImpact:
UpgradeImpact:
Merge "Update migration defaults"