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.

src/libxl/libxl_driver.c

@@ -5259,6 +5259,7 @@ libxlDomainGetJobStats(virDomainPtr dom,
     return ret;
 }
 
+#ifdef __linux__
 static int
 libxlDiskPathToID(const char *virtpath)
 {
@@ -5348,7 +5349,6 @@ libxlDiskSectorSize(int domid, int devno)
     return ret;
 }
 
-#ifdef __linux__
 static int
 libxlDomainBlockStatsVBD(virDomainObjPtr vm,
                          const char *dev,