Introduce the code to handle xenbus state changes.

Implement the probe function for the pvcalls backend. Write the
supported versions, max-page-order and function-calls nodes to xenstore,
as required by the protocol.

Introduce stub functions for disconnecting/connecting to a frontend.
Signed-off-by: NStefano Stabellini <stefano@aporeto.com>
Reviewed-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
Signed-off-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>