libxl: remove reference to non-existent out label

Commit e6336442 changed the 'out:' label to 'cleanup'  in
libxlDomainAttachNetDevice(), but missed a comment referencing
the 'out:' label. Remove it from the comment since it is no
longer accurate anyhow.

src/libxl/libxl_driver.c

@@ -3159,7 +3159,7 @@ libxlDomainAttachNetDevice(libxlDriverPrivatePtr driver,
         /* This is really a "smart hostdev", so it should be attached
          * as a hostdev (the hostdev code will reach over into the
          * netdev-specific code as appropriate), then also added to
-         * the nets list (see out:) if successful.
+         * the nets list if successful.
          */
         ret = libxlDomainAttachHostDevice(driver, vm, hostdev);
         goto cleanup;