libqtest: Fix possible deadlock in qtest initialization

'socket_accept' waits for QEMU to init its unix socket.
If QEMU encounters an error during command line parsing,
it can exit before initializing the communication channel.

Using a timeout for sockets fixes the issue.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
