Limit them to the device_add functionality.  Device aliases were a hack based
on the fact that virtio was modeled the wrong way.  The mechanism for aliasing
is very limited in that only one alias can exist for any device.

We have to support it for the purposes of compatibility but we only need to
support it in device_add so restrict it to that piece of code.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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v1 -> v2
 - Use a table for aliases (Paolo)