libxl: libxl__xs_write format string should be const.

libxl: libxl__xs_write format string should be const.

George Dunlap reports that gcc 4.4.3 complains:
  libxl_dm.c: In function libxl__create_device_mode:
  libxl_dm.c:776: error: format not a string literal and no format arguments
And indeed the format argument here is a char * from libxl__domain_bios().

Make the argument to libxl__xs_write a const char * and change
libxl__domain_bios to return a const char too.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>