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: NJuan Quintela <quintela@redhat.com>

block-migration.c

@@ -548,13 +548,6 @@ static int block_save_live(QEMUFile *f, int stage, void *opaque)
     DPRINTF("Enter save live stage %d submitted %d transferred %d\n",
             stage, block_mig_state.submitted, block_mig_state.transferred);
 
-
-    if (block_mig_state.blk_enable != 1) {
-        /* no need to migrate storage */
-        qemu_put_be64(f, BLK_MIG_FLAG_EOS);
-        return 1;
-    }
-
     if (stage == 1) {
         init_blk_migration(f);
 
@@ -710,11 +703,17 @@ static void block_set_params(const MigrationParams *params, void *opaque)
     block_mig_state.blk_enable |= params->shared;
 }
 
+static bool block_is_active(void *opaque)
+{
+    return block_mig_state.blk_enable == 1;
+}
+
 SaveVMHandlers savevm_block_handlers = {
     .set_params = block_set_params,
     .save_live_state = block_save_live,
     .load_state = block_load,
     .cancel = block_migration_cancel,
+    .is_active = block_is_active,
 };
 
 void blk_mig_init(void)

savevm.c

@@ -1576,6 +1576,11 @@ int qemu_savevm_state_begin(QEMUFile *f,
         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 @@ int qemu_savevm_state_iterate(QEMUFile *f)
         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 @@ int qemu_savevm_state_complete(QEMUFile *f)
         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);

vmstate.h

@@ -35,6 +35,7 @@ typedef struct SaveVMHandlers {
     int (*save_live_state)(QEMUFile *f, int stage, void *opaque);
     void (*cancel)(void *opaque);
     LoadStateHandler *load_state;
+    bool (*is_active)(void *opaque);
 } SaveVMHandlers;
 
 int register_savevm(DeviceState *dev,