iothread: add iothread_by_id() API

Encapsulate IOThread QOM object lookup so that callers don't need to
know how and where IOThread objects live.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20171206144550.22295-8-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/sysemu/iothread.h

@@ -42,6 +42,7 @@ typedef struct {
    OBJECT_CHECK(IOThread, obj, TYPE_IOTHREAD)
 
 char *iothread_get_id(IOThread *iothread);
+IOThread *iothread_by_id(const char *id);
 AioContext *iothread_get_aio_context(IOThread *iothread);
 void iothread_stop_all(void);
 GMainContext *iothread_get_g_main_context(IOThread *iothread);

iothread.c

@@ -380,3 +380,10 @@ void iothread_destroy(IOThread *iothread)
 {
     object_unparent(OBJECT(iothread));
 }
+
+/* Lookup IOThread by its id.  Only finds user-created objects, not internal
+ * iothread_create() objects. */
+IOThread *iothread_by_id(const char *id)
+{
+    return IOTHREAD(object_resolve_path_type(id, TYPE_IOTHREAD, NULL));
+}