Instead of doing I/O one-byte-at-a-time in lxc_console,
which is slow, let's do it in batches.  Only for output
(from container to the host system), since input is most
likely one-byte-at-a-time anyway (from a keyboard).
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>