tools/xenstat: Fix -Wformat-truncation= issue

tools/xenstat: Fix -Wformat-truncation= issue

Building with GCC 8.3 on Buster identifies:

  src/xenstat_linux.c: In function 'xenstat_collect_networks':
  src/xenstat_linux.c:307:32: warning: 'snprintf' output may be truncated before
  the last format character [-Wformat-truncation=]
    snprintf(devNoBridge, 16, "p%s", devBridge);
                                  ^
  src/xenstat_linux.c:307:2: note: 'snprintf' output between 2 and 17 bytes into
  a destination of size 16
    snprintf(devNoBridge, 16, "p%s", devBridge);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

devNoBridge[] needs one charater more than devBridge[], so allocate one byte
more.  Replace a raw 16 in the snprintf() call with a sizeof() expression
instead.

Finally, libxenstat, unlike most of the rest of the Xen, doesn't use -Werror
which is why this issue went unnoticed in CI.  Fix this.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>