migration: Don't sync volatile memory after migration completes

migration: Don't sync volatile memory after migration completes

Syncing volatile memory provides no benefit, instead it can cause
performance issues in some cases.  Only sync memory that is marked as
non-volatile after migration completes on destination.

Signed-off-by: Ben Chaney <bchaney@akamai.com>
Fixes: bd108a44bc29 (migration: ram: Switch to ram block writeback)
Link: https://lore.kernel.org/r/1CC43F59-336F-4A12-84AD-DB89E0A17A95@akamai.com
Signed-off-by: Peter Xu <peterx@redhat.com>
(cherry picked from commit 983899eab4939dc4dff67fa4d822c5b4df7eae21)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>