hw/riscv: virt: Use machine->ram as the system memory

hw/riscv: virt: Use machine->ram as the system memory

If default main_mem is used to be registered as the system memory,
other memory cannot be initialized. Therefore, the system memory
should be initialized to the machine->ram, which consists of the
default main_mem and other possible memory required by applications,
such as shared hugepage memory in DPDK.

Also, the mc->defaul_ram_id should be set to the default main_mem,
such as "riscv_virt_board.ram" for the virt machine.

Signed-off-by: Mingwang Li <limingwang@huawei.com>
Signed-off-by: Yifei Jiang <jiangyifei@huawei.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211016030908.40480-1-limingwang@huawei.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>