tools/xenpmd: fix possible '\0' truncation

tools/xenpmd: fix possible '\0' truncation

gcc-8 complains:
    xenpmd.c:207:9: error: 'strncpy' specified bound 32 equals destination size [-Werror=stringop-truncation]
             strncpy(info->oem_info, attrib_value, 32);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    xenpmd.c:201:9: error: 'strncpy' specified bound 32 equals destination size [-Werror=stringop-truncation]
             strncpy(info->battery_type, attrib_value, 32);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    xenpmd.c:195:9: error: 'strncpy' specified bound 32 equals destination size [-Werror=stringop-truncation]
             strncpy(info->serial_number, attrib_value, 32);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    xenpmd.c:189:9: error: 'strncpy' specified bound 32 equals destination size [-Werror=stringop-truncation]
             strncpy(info->model_number, attrib_value, 32);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Copy 31 chars, then make sure terminating '\0' is present. Those fields
are passed to strlen and as '%s' for snprintf later.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>