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: bd108a44bc (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>

migration/ram.c

@@ -3680,8 +3680,10 @@ static int ram_load_cleanup(void *opaque)
     RAMBlock *rb;
 
     RAMBLOCK_FOREACH_NOT_IGNORED(rb) {
+        if (memory_region_is_nonvolatile(rb->mr)) {
             qemu_ram_block_writeback(rb);
         }
+    }
 
     xbzrle_load_cleanup();