xen/xenbus-frontend: Make error message more clear

Add the work frontend to the error message because we now also have a
backend device.

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>