Deprecation period is run out and it's a time to flip the switch
introduced by cd5ff833.  Disable legacy option for new machine
types (since 5.1) and amend documentation.

'-numa node,memdev' shall be used instead of disabled option
with new machine types.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20200609135635.761587-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>