When writing out the CRIU exec command, we're bumping up against the buffer
size limit. Let's increase it so we can avoid:

lxc 20160509213229.921 WARN     lxc_log - log.c:log_append_logfile:111 - truncated next event from 523 to 512 bytes
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>