multifd: Change default packet size

We moved from 64KB to 512KB, as it makes less locking contention
without any downside in testing.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -584,7 +584,7 @@ exit:
 #define MULTIFD_FLAG_SYNC (1 << 0)
 
 /* This value needs to be a multiple of qemu_target_page_size() */
-#define MULTIFD_PACKET_SIZE (64 * 1024)
+#define MULTIFD_PACKET_SIZE (512 * 1024)
 
 typedef struct {
     uint32_t magic;