migration: Move ram_stats to its own file migration-stats.[ch]

There is already include/qemu/stats.h, so stats.h was a bad idea.
We want this file to not depend on anything else, we will move all the
migration counters/stats to this struct.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>

migration/savevm.c

@@ -31,6 +31,7 @@
 #include "net/net.h"
 #include "migration.h"
 #include "migration/snapshot.h"
+#include "migration-stats.h"
 #include "migration/vmstate.h"
 #include "migration/misc.h"
 #include "migration/register.h"