qemu: Fill default value in //cpu/@migratable attribute

qemu: Fill default value in //cpu/@migratable attribute

Before QEMU introduced migratable CPU property, "-cpu host" included all
features that could be enabled on the host, even those which would block
migration. In other words, the default was equivalent to migratable=off.
When the migratable property was introduced, the default changed to
migratable=on. Let's record the default in domain XML.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>