ppc: Use a larger default memory size on ppc64.

Without this, test-max-disks fails when creating device nodes.
ppc: Disallow ide interface.

qemu-system-ppc64 lets you add the IDE interface to the appliance, but
the appliance kernel ignores it (and consequently can't find the
appliance disk so it all goes wrong).  Best to just disallow this.

Also skip tests that try using iface = 'ide'.