Fix leak in hotplug code in QEMU driver

The hotplug code in QEMU was leaking memory because although the
inner device object was being moved into the main virDomainDefPtr
config object, the outer container virDomainDeviceDefPtr was not.

 * src/qemu/qemu_driver.c: Clarify code to show that the inner
   device object is owned by the main domain config upon
   successfull attach.