qemu: Properly update completed migration stats

qemu: Properly update completed migration stats

We should not overwrite all migration statistics on the source with the
numbers sent by the destination since the source may have an updated
view in some cases (such as post-copy migration). It's safer to update
just the timing info we need to get from the destination and be prepared
for the future. And we should only do all this after a successful
migration.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>