xen/netfront: select XEN_XENBUS_FRONTEND

Make sure the Xen frontend xenbus is enabled.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[corresponds to c40912891c3b in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git

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