hw/arm/virt: Wire up memory region to CPUs explicitly

hw/arm/virt: Wire up memory region to CPUs explicitly

Wire up the system memory region to the CPUs explicitly
by setting the QOM property. This doesn't change anything
over letting it default, but will be needed for adding
a secure memory region later.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>