tools/libxl: remove usage of VLA arrays

Clang 19 complains with the following error when building libxl:

libxl_utils.c:48:15: error: variable length array folded to constant array as an extension [-Werror,-Wgnu-folding-constant]
   48 |     char path[strlen("/local/domain") + 12];
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Replace the usage of strlen() with sizeof, which allows the literal
string length to be known at build time.  Note sizeof accounts for the
NUL terminator while strlen() didn't, hence subtract 1 from the total
size calculation.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>
(cherry picked from commit a7c7c3f642)

tools/libs/light/libxl_utils.c

@@ -45,7 +45,7 @@ unsigned long libxl_get_required_shadow_memory(unsigned long maxmem_kb, unsigned
 char *libxl_domid_to_name(libxl_ctx *ctx, uint32_t domid)
 {
     unsigned int len;
-    char path[strlen("/local/domain") + 12];
+    char path[sizeof("/local/domain") + 11];
     char *s;
 
     snprintf(path, sizeof(path), "/local/domain/%d/name", domid);
@@ -141,7 +141,7 @@ int libxl_cpupool_qualifier_to_cpupoolid(libxl_ctx *ctx, const char *p,
 char *libxl_cpupoolid_to_name(libxl_ctx *ctx, uint32_t poolid)
 {
     unsigned int len;
-    char path[strlen("/local/pool") + 12];
+    char path[sizeof("/local/pool") + 11];
     char *s;
 
     snprintf(path, sizeof(path), "/local/pool/%d/name", poolid);