migration: Split save_live_pending() into state_pending_*

We split the function into to:

- state_pending_estimate: We estimate the remaining state size without
  stopping the machine.

- state pending_exact: We calculate the exact amount of remaining
  state.

The only "device" that implements different functions for _estimate()
and _exact() is ram.

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

docs/devel/migration.rst

@@ -482,15 +482,17 @@ An iterative device must provide:
   - A ``load_setup`` function that initialises the data structures on the
     destination.
 
-  - A ``save_live_pending`` function that is called repeatedly and must
-    indicate how much more data the iterative data must save.  The core
-    migration code will use this to determine when to pause the CPUs
-    and complete the migration.
+  - A ``state_pending_exact`` function that indicates how much more
+    data we must save.  The core migration code will use this to
+    determine when to pause the CPUs and complete the migration.
 
-  - A ``save_live_iterate`` function (called after ``save_live_pending``
-    when there is significant data still to be sent).  It should send
-    a chunk of data until the point that stream bandwidth limits tell it
-    to stop.  Each call generates one section.
+  - A ``state_pending_estimate`` function that indicates how much more
+    data we must save.  When the estimated amount is smaller than the
+    threshold, we call ``state_pending_exact``.
+
+  - A ``save_live_iterate`` function should send a chunk of data until
+    the point that stream bandwidth limits tell it to stop.  Each call
+    generates one section.
 
   - A ``save_live_complete_precopy`` function that must transmit the
     last section for the device containing any remaining data.