Introduce a xenbus backend for the pvcalls protocol, as defined by
https://xenbits.xen.org/docs/unstable/misc/pvcalls.html.

This patch only adds the stubs, the code will be added by the following
patches.
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>