libxl: only return {OVMF,SEABIOS}_PATH if available

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_paths.c

@@ -37,12 +37,20 @@ const char *libxl__run_dir_path(void)
 
 const char *libxl__seabios_path(void)
 {
+#ifdef SEABIOS_PATH
     return SEABIOS_PATH;
+#else
+    return NULL;
+#endif
 }
 
 const char *libxl__ovmf_path(void)
 {
+#ifdef OVMF_PATH
     return OVMF_PATH;
+#else
+    return NULL;
+#endif
 }
 
 /*