multifd: Create property multifd-flush-after-each-section

We used to flush all channels at the end of each RAM section
sent.  That is not needed, so preparing to only flush after a full
iteration through all the RAM.

Default value of the property is false.  But we return "true" in
migrate_multifd_flush_after_each_section() until we implement the code
in following patches.

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

---

Rename each-iteration to after-each-section
Rename multifd-sync-after-each-section to
       multifd-flush-after-each-section
Move to machine-8.0 (peter)

migration/options.c

@@ -88,6 +88,8 @@ Property migration_properties[] = {
                      send_section_footer, true),
     DEFINE_PROP_BOOL("decompress-error-check", MigrationState,
                       decompress_error_check, true),
+    DEFINE_PROP_BOOL("multifd-flush-after-each-section", MigrationState,
+                      multifd_flush_after_each_section, true),
     DEFINE_PROP_UINT8("x-clear-bitmap-shift", MigrationState,
                       clear_bitmap_shift, CLEAR_BITMAP_SHIFT_DEFAULT),
     DEFINE_PROP_BOOL("x-preempt-pre-7-2", MigrationState,
@@ -335,6 +337,17 @@ bool migrate_zero_copy_send(void)
 
 /* pseudo capabilities */
 
+bool migrate_multifd_flush_after_each_section(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    /*
+     * Until the patch that remove this comment, we always return that
+     * the property is enabled.
+     */
+    return true || s->multifd_flush_after_each_section;
+}
+
 bool migrate_postcopy(void)
 {
     return migrate_postcopy_ram() || migrate_dirty_bitmaps();