qemu: hotplug: Fix error reported when cdrom tray is locked

Commit 1fad65d49a used a really big hammer
and overwrote the error message that might be reported by qemu if the
tray is locked. Fix it by reporting the error only if no error is
currently set.

Error after commit mentioned above:
error: internal error: timed out waiting for disk tray status update

New error:
error: internal error: unable to execute QEMU command 'eject': Tray of
device 'drive-ide0-0-0' is not open

src/qemu/qemu_hotplug.c

@@ -163,8 +163,12 @@ qemuHotplugWaitForTrayEject(virQEMUDriverPtr driver,
             return -1;
 
         if (rc > 0) {
-            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                           _("timed out waiting for disk tray status update"));
+            /* the caller called qemuMonitorEjectMedia which usually reports an
+             * error. Report the failure in an off-chance that it didn't. */
+            if (!virGetLastError()) {
+                virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                               _("timed out waiting for disk tray status update"));
+            }
             return -1;
         }
     }