xl: fix compiler warnings

xl: fix compiler warnings

either GCC 4.6.1 or Ubuntu add -Werror=format-security to the -Wall set,
so libxl compilation breaks:
   libxl_create.c: In function 'store_libxl_entry':
   libxl_create.c:454:9: error: format not a string literal and no
format arguments [-Werror=format-security]
   cc1: all warnings being treated as errors

Attached patch fixes this and another occurrence.

Patch from: Uwe Dannowski

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>