When attaching a device that already exists, xend driver updates
the device with "device_configure", it causes problems (e.g. for
disk device, 'device_configure' only can be used to update device
like CDROM), on the other hand, we provide additional API
(virDomainUpdateDevice) to update device, this fix is to raise up
errors instead of updating the existed device which is not CDROM
device.

Changes from v1 to v2:
  - allow to update CDROM

* src/xen/xend_internal.c