Enable live block migration for libvirt.

This works around Nova's defaults not enabling block migration, but
manual testing showed it working well here.

See also I7b997666474ec324c5034e3660a7271b8423bca8 in nova which will
change the defaults to this.

Change-Id: Ia3d116c6974d1ec4f3fc0add28b1684be7ed59ff
Merge "Enable live block migration for libvirt."