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

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>