qemuMonitorJSONEjectMedia: don't stringify the replay at all

Commit 256496e1 introduced a detection if "is locked" in error replay
from qemu monitor. Commit c4073657 fixed a memory leak, but it was
pointed out by Peter, that this could be done cleaner without
stringifing the replay.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -2223,11 +2223,12 @@ int qemuMonitorJSONEjectMedia(qemuMonitorPtr mon,
         ret = qemuMonitorJSONCheckError(cmd, reply);
 
     if (ret < 0) {
-        char *replyStr = virJSONValueToString(reply, false);
-
-        if (c_strcasestr(replyStr, "is locked"))
-            ret = -2;
-        VIR_FREE(replyStr);
+        virJSONValuePtr error = virJSONValueObjectGet(reply, "error");
+        if (error) {
+            const char *errorStr = virJSONValueObjectGetString(error, "desc");
+            if (errorStr && c_strcasestr(errorStr, "is locked"))
+                ret = -2;
+        }
     }
 
     virJSONValueFree(cmd);