Xenstore user-space device has two problems currently:
- It does not correctly handle concurrent clients, because it's storing
each client data in dev->si_drv1.
- It does not correctly free this data when the client closes the device.
In order to solve both of this issues store the per-client data using
cdevpriv, which also comes with a hook in order to perform the necessary
cleanup on device close.
While there also make the device eternal.
Sponsored by: Citrix Systems R&D
Reported and Tested by: thompsa
MFC after: 2 weeks
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