libxl: Move bdf parsing into libxlu

libxl: Move bdf parsing into libxlu

Config parsing functions do not properly belong in libxl.  Move them into
libxlu so that others can use them or not as they see fit.

No functional changes.  One side-effect was making public a private libxl
utility function which just set the elements of a structure from the  function
arguments passed in.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>