xen/pvcalls: handle commands from the frontend

When the other end notifies us that there are commands to be read
(pvcalls_back_event), wake up the backend thread to parse the command.

The command ring works like most other Xen rings, so use the usual
ring macros to read and write to it. The functions implementing the
commands are empty stubs for now.

[ boris: fixed whitespaces ]

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>