Fix toogle whether xenbus driver should be built as module or part of kernel.

The "select" statement in the Kconfig ensures that all other users
of the xenbus will have the same dependency.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>