For QEMU PPC64 we have a machine type ("pseries") which has a virtual
bus called "spapr-vio". We need to be able to create devices on this
bus, and as such need a way to specify the address for those devices.

This patch adds a new address type "spapr-vio", which achieves this.

The addressing is specified with a "reg" property in the address
definition. The reg is optional, if it is not specified QEMU will
auto-assign an address for the device.
Signed-off-by: NMichael Ellerman <michael@ellerman.id.au>