pc: add 2.1 machine type

At the moment, 2.1 and 2.0 machines are identical.
As several people are working on incompatible changes
to the PC machine, collaboration will be made easier
by merging this place-holder.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>