xen/xenbus: clean up error handling

Don't report errors when booting on non-Xen, because its just confusing.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[corresponds to 8aa08376d6aa in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git

]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>