vhost.m4: Refactor virtual host routing some more.

Move the does-this-key-exist condition out of the `VHOST' macro and
into `VHOST_FILTER'.  Alas, this means we must repeat the logic for
the `virtual_route' router.  But we can take the opportunity to elide
the unnecessary existence checks for the domain keys we just tested in
the router conditions.

Also, switch the arguments to `VHOST_FILTER' around.  I think they
make more sense like this.