fix fdleak and errors in lxc_create_tty()

fix fdleak and errors in lxc_create_tty()

if, for some reason, openpty() fails, lxc_create_tty() will
leak all previous ptys and leave the config structure in a
inconsistent state (wrt the number of ptys actually opened)
Fix that by explicitly closing all previously opened ptys
in case of failure and by setting number of actually opened
ttys after actual open

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>