Move 'upgrade' device logic to xenopsd

[djs55]

I believe the upgrade thing is really a policy decision -- we decided which devices we wanted to use in future. I think it's better if the Device_number module focuses on the type:

• what is a device number
• how can one be created
• what properties can one have (in theory, even if not all are currently used)
• how do they correspond to Xenstore devids

So I think we should move the 'upgrade_device_number' function to the caller in Xenopsd, if possible.

[djs55]

Furthermore the functions which take an 'hvm: bool' argument should have the argument removed, if we are now treating PV and HVM domains the same.