vhost-user-test: use tmpfs by default

Most people don't run make check by default, so they skip vhost-user
unit tests.  Solve this by using tmpfs instead, unless hugetlbfs is
specified (using an environment variable).

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