migration: Rename res_{postcopy,precopy}_only

Once that res_compatible is removed, they don't make sense anymore.
We remove the _only preffix.  And to make things clearer we rename
them to must_precopy and can_postcopy.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index 9c6655e..fe73aa9 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -763,8 +763,8 @@
 }
 
 static void dirty_bitmap_state_pending(void *opaque,
-                                       uint64_t *res_precopy_only,
-                                       uint64_t *res_postcopy_only)
+                                       uint64_t *must_precopy,
+                                       uint64_t *can_postcopy)
 {
     DBMSaveState *s = &((DBMState *)opaque)->save;
     SaveBitmapState *dbms;
@@ -784,7 +784,7 @@
 
     trace_dirty_bitmap_state_pending(pending);
 
-    *res_postcopy_only += pending;
+    *can_postcopy += pending;
 }
 
 /* First occurrence of this bitmap. It should be created if doesn't exist */