Add a new config option for all backend drivers. This has two benefits:

Add a new config option for all backend drivers. This has two benefits:
 1. All backend drivers can be disabled or modularised via
    one config option.
 2. Backend helper routines that are not specific to any particular driver
    can be disabled or modularised based on this config option. In
    particular this may allow backend drivers plus the service module
    to be upgraded separate from the kernel core as and when the backend
    interfaces change (and they will).

Signed-off-by: Keir Fraser <keir@xensource.com>