libxl: fix unused functions

libxl: fix unused functions

Commit eee7bd4e introduced two functions: libxlDiskPathToID and
libxlDiskSectorSize.

However, as they're used only by code under #ifdef __linux__,
on non-Linux platforms it results in errors similar to this:

 CC       libxl/libvirt_driver_libxl_impl_la-libxl_driver.lo
libxl/libxl_driver.c:5263:1: error: unused function 'libxlDiskPathToID' [-Werror,-Wunused-function]
libxlDiskPathToID(const char *virtpath)
^
libxl/libxl_driver.c:5312:1: error: unused function 'libxlDiskSectorSize' [-Werror,-Wunused-function]
libxlDiskSectorSize(int domid, int devno)
^
2 errors generated.

Fix that by moving these functions under the #ifdef __linux__ block.