Replaced the device handling mechanism used by XendDomainInfo. Superficially,

Replaced the device handling mechanism used by XendDomainInfo.  Superficially,
this looks like the resurrection of DevController from controller.py, but the
mechanism is actually very different.  Device handling is now stateless inside
xend, relying on the store for state management, reducing DevController
instances to one-shot lifetimes.  Dev and its subclasses have gone completely.
The device creation code itself has moved from XendDomainInfo.createDevice into
DevController subclasses, and the previous contents of the subclasses has been
discarded (these subclasses were no longer being called, so all this code was
dead).

XendDomainInfo.getDeviceIds has gone, as it was unused.
XendDomainInfo.delete_device has been subsumed by XendDomainInfo.destroyDevice;
since device handling is now stateless inside xend, the distinction between
device 'deletion' and device 'destruction' is meaningless.

The s-expression describing devices has gone, as this information is no longer
available to xend in the same way, and seems to be unused.  If it is required,
it can be reinstated by loading device information from Xen or the store.

Signed-off-by: Ewan Mellor <ewan@xensource.com>