ppc: parse cpu features once

Considering that features are converted to global properties and
global properties are automatically applied to every new instance
of created CPU (at object_new() time), there is no point in
parsing cpu_model string every time a CPU created. So move
parsing outside CPU creation loop and do it only once.

Parsing also should be done before any CPU is created so that
features would affect the first CPU a well.

This patch does that for all PowerPC machine types.

It is based on previous work from Bharata:

https://lists.nongnu.org/archive/html/qemu-devel/2016-06/msg07564.html

Signed-off-by: Greg Kurz <groug@kaod.org>
[clg: only kept the fix for the spapr platform. support for other
      platform will be added in 2.8 ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
