qom/cpu: Add MemoryRegion property

qom/cpu: Add MemoryRegion property

Add a MemoryRegion property, which if set is used to construct
the CPU's initial (default) AddressSpace.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
[PMM: code is moved from qom/cpu.c to exec.c to avoid having to
 make qom/cpu.o be a non-common object file; code to use the
 MemoryRegion and to default it to system_memory added.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>