savevm: introduce is_active method

Enable the creation of a method to tell migration if that section is
active and should be migrate.  We use it for blk-migration, that is
normally not active.  We don't create the method for RAM, as setups
without RAM are very strange O:-)

Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/savevm.c b/savevm.c
index 888c5a2..afa0c9e 100644
--- a/savevm.c
+++ b/savevm.c
@@ -1576,6 +1576,11 @@
         if (!se->ops || !se->ops->save_live_state) {
             continue;
         }
+        if (se->ops && se->ops->is_active) {
+            if (!se->ops->is_active(se->opaque)) {
+                continue;
+            }
+        }
         /* Section type */
         qemu_put_byte(f, QEMU_VM_SECTION_START);
         qemu_put_be32(f, se->section_id);
@@ -1618,6 +1623,11 @@
         if (!se->ops || !se->ops->save_live_state) {
             continue;
         }
+        if (se->ops && se->ops->is_active) {
+            if (!se->ops->is_active(se->opaque)) {
+                continue;
+            }
+        }
         if (qemu_file_rate_limit(f)) {
             return 0;
         }
@@ -1658,6 +1668,11 @@
         if (!se->ops || !se->ops->save_live_state) {
             continue;
         }
+        if (se->ops && se->ops->is_active) {
+            if (!se->ops->is_active(se->opaque)) {
+                continue;
+            }
+        }
         trace_savevm_section_start();
         /* Section type */
         qemu_put_byte(f, QEMU_VM_SECTION_END);