docs: virtiofs: move legacy docs to the bottom

docs: virtiofs: move legacy docs to the bottom

A number of legacy issues make the virtiofs kbase article hard to
understand. Most users don't need to configure NUMA or a memory backend
other than memfd. Move that information to the bottom of the article so
the recommended syntax is most prominent.

Suggested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>