xenbus/frontend: register bus earlier

Impact: bugfix

Make sure the frontend bus is registered early, before
any drivers want to attach.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[corresponds to 1d5df318f87f in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git

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