xen: Add module alias to autoload backend drivers

All the Xen backend drivers are assigned to a special bus type
xen-backend. This patch exports xen-backend:* names through modalias and
uevent to autoload them.

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>