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>

tests/libqtest.c

@@ -74,6 +74,7 @@ static int socket_accept(int sock)
     socklen_t addrlen;
     int ret;
 
+    addrlen = sizeof(addr);
     do {
         ret = accept(sock, (struct sockaddr *)&addr, &addrlen);
     } while (ret == -1 && errno == EINTR);