qtest: Be paranoid about accept() addrlen argument

POSIX specifies that address_len shall on output specify the length of
the stored address; it does not however specify whether it may get
updated on failure as well to, e.g., zero.

In case EINTR occurs, re-initialize the variable to the desired value.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
