libqtest: Fix socket_accept() to pass address_len

accept() expects address_len to point to the length of the sockaddr on
input. Initialize it accordingly.

Resolves an assertion due to EFAULT on illumos.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
