migration: introduce 'background-snapshot' migration capability

Add new capability to 'qapi/migration.json' schema.
Update migrate_caps_check() to validate enabled capability set
against introduced one. Perform checks for required kernel features
and compatibility with guest memory backends.

Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210129101407.103458-2-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -3788,6 +3788,27 @@ static int ram_resume_prepare(MigrationState *s, void *opaque)
     return 0;
 }
 
+/* ram_write_tracking_available: check if kernel supports required UFFD features
+ *
+ * Returns true if supports, false otherwise
+ */
+bool ram_write_tracking_available(void)
+{
+    /* TODO: implement */
+    return false;
+}
+
+/* ram_write_tracking_compatible: check if guest configuration is
+ *   compatible with 'write-tracking'
+ *
+ * Returns true if compatible, false otherwise
+ */
+bool ram_write_tracking_compatible(void)
+{
+    /* TODO: implement */
+    return false;
+}
+
 static SaveVMHandlers savevm_ram_handlers = {
     .save_setup = ram_save_setup,
     .save_live_iterate = ram_save_iterate,