spapr: Cleanup machine naming conventions, and prepare for 2.2 release

As of qemu-2.1, spapr/pseries, has a set of versioned machine classes to
represent the machine type as it appeared to the guest in different qemu
versions.  This allows for safe migration of guests between current and
future qemu versions.

However, these are organized a bit differently from those for PC: on PC,
the default plain "pc" machine type is just an alias for the most recent
versioned machine type.  In sPAPR, it names the base machine class from
which the versioned types are derived.

The PC approach is preferable; it makes it clearer which explicit version
is the current one.  Additionally updating the "current" machine as the
base class makes it even more likely than otherwise to incorrectly alter
the versioned machines' behaviour when updating the current machine.

Therefore this patch changes sPAPR to the PC approach - the base class
becomes abstract, and plain "pseries" becomes an alias for the most
recent versioned machine class.  Since qemu-2.1 is now released, we also
create a new pseries-2.2 machine type, to incorporate changes during this
development cycle (for now it is identical to pseries-2.1).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>