xen: Add xenbus_backend device

Access for xenstored to the event channel and pre-allocated ring is
managed via xenfs.  This adds its own character device featuring mmap
for the ring and an ioctl for the event channel.

Signed-off-by: Bastian Blank <waldi@debian.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>