hw/riscv: sifive_u: Use MachineState::ram and MachineClass::default_ram_id

hw/riscv: sifive_u: Use MachineState::ram and MachineClass::default_ram_id

Using memory_region_init_ram(), which can't possibly handle vhost-user,
and can't work as expected with '-numa node,memdev' options.

Use MachineState::ram instead of manually initializing RAM memory
region, as well as by providing MachineClass::default_ram_id to
opt in to memdev scheme.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211020014112.7336-6-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>