spapr: Rename machine init functions for clarity

Machine objects have two init functions - the generic QOM level
instance_init which should only do static object initialization, and
the Machine specific MachineClass::init which does the actual
construction of the machine.

In spapr the functions implementing these two have names -
ppc_machine_initfn() and ppc_spapr_init() - which don't correspond closely
to either of those.  To prevent people (read, me) from confusing which is
which, rename them spapr_instance_init() and spapr_machine_init() to
make it clearer which is which.

While we're there rename ppc_spapr_reset() to spapr_machine_reset() to
match.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>