Add a function to find a bit in the radix tree for a given
pfn. Also add code to the memory bitmap wrapper functions to
use the radix tree together with the existing memory bitmap
implementation.

On read accesses compare the results of both bitmaps to make
sure the radix tree behaves the same way.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>