vhost-user-test: fix predictable filename on tmpfs

vhost-user-test uses getpid to create a unique filename. This name is
predictable, and a security problem.  Instead, use a tmp directory
created by mkdtemp, which is a suggested best practice.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
