target-ppc: Change default machine for 64-bit

Currently, for qemu-system-ppc64, the default machine type is 'mac99'.
Since the mac99 machine is not being actively maintained, and shows quite
a few signs of bitrot, this is not very sensible.  This patch changes the
default machine to 'pseries', which is actively maintained and works well
with most modern ppc64 Linux distributions as a guest.

Because the pseries machine type is optional (it is only built when libfdt
is available), this can result in a build with no default machine.  In that
case vl.c will print a "No machine found" message.  This seems reasonable,
given that as mentioned, mac99 is unlikely to be a good choice.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>