xen: Populate xenbus device attributes

The xenbus bus type uses device_create_file to assign all used device
attributes. However it does not remove them when the device goes away.

This patch uses the dev_attrs field of the bus type to specify default
attributes for all devices.

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>