target-ppc: Change default machine for 64-bit

target-ppc: Change default machine for 64-bit

Currently, for qemu-system-ppc64, the default machine type is 'mac99'.
The mac99 machine is not being actively maintained, and represents a
bizarre hybrid of components that never actually existed as a real system.
This patch changes the default machine to 'pseries', which is actively
maintained and works well with most modern ppc64 Linux distributions as a
guest.

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