hw/riscv: virt: Limit RAM size in a 32-bit system

hw/riscv: virt: Limit RAM size in a 32-bit system

RV32 supports 34-bit physical address hence the maximum RAM size
should be limited. Limit the RAM size to 10 GiB, which leaves
some room for PCIe high mmio space.

For 32-bit host, this is not needed as machine->ram_size cannot
represent a RAM size that big. Use a #if size test to only do
the size limitation for the 64-bit host.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210220144807.819-4-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>