tools: fix several "format-truncation" warnings with GCC 7

GCC 7.1.1 complains that several buffers passed to snprintf() in xenpmd
and tools/ocmal/xc are too small to hold the largest possible resulting string,
which is calculated by adding up the maximum length of all the substrings.

The warnings are treated as errors by -Werror, and goes like this (abbreviated):

xenpmd.c:94:36: error: ‘%s’ directive output may be truncated writing up to
255 bytes into a region of size 13 [-Werror=format-truncation=]
     #define BATTERY_INFO_FILE_PATH "/proc/acpi/battery/%s/info"
                                    ^
xenpmd.c:113:13: note: ‘snprintf’ output between 25 and 280 bytes into a
destination of size 32

xenpmd.c:95:37: error: ‘%s’ directive output may be truncated writing up to
255 bytes into a region of size 13 [-Werror=format-truncation=]
     #define BATTERY_STATE_FILE_PATH "/proc/acpi/battery/%s/state"
                                     ^
xenpmd.c:116:13: note: ‘snprintf’ output between 26 and 281 bytes into a
destination of size 32

xenctrl_stubs.c:65:15: error: ‘%s’ directive output may be truncated writing
up to 1023 bytes into a region of size 252 [-Werror=format-truncation=]
      "%d: %s: %s", error->code,
               ^~
xenctrl_stubs.c:64:4: note: ‘snprintf’ output 5 or more bytes (assuming 1028)
into a destination of size 256

Enlarge the size of these buffers as suggested by the complier
(and slightly rounded) to fix the warnings.

No functional changes.

Signed-off-by: Zhongze Liu <blackskygg@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>
(cherry picked from commit 2d78f78a14)

tools/ocaml/libs/xc/xenctrl_stubs.c

@@ -54,7 +54,7 @@
 
 static void Noreturn failwith_xc(xc_interface *xch)
 {
-	char error_str[256];
+	char error_str[1028];
 	if (xch) {
 		const xc_error *error = xc_get_last_error(xch);
 		if (error->code == XC_ERROR_NONE)

tools/xenpmd/xenpmd.c

@@ -100,7 +100,7 @@ FILE *get_next_battery_file(DIR *battery_dir,
 {
     FILE *file = 0;
     struct dirent *dir_entries;
-    char file_name[32];
+    char file_name[284];
     
     do 
     {
@@ -110,10 +110,10 @@ FILE *get_next_battery_file(DIR *battery_dir,
         if ( strlen(dir_entries->d_name) < 4 )
             continue;
         if ( battery_info_type == BIF ) 
-            snprintf(file_name, 32, BATTERY_INFO_FILE_PATH,
+            snprintf(file_name, sizeof(file_name), BATTERY_INFO_FILE_PATH,
                      dir_entries->d_name);
         else 
-            snprintf(file_name, 32, BATTERY_STATE_FILE_PATH,
+            snprintf(file_name, sizeof(file_name), BATTERY_STATE_FILE_PATH,
                      dir_entries->d_name);
         file = fopen(file_name, "r");
     } while ( !file );