hw/riscv: virt: Add a machine done notifier

hw/riscv: virt: Add a machine done notifier

Move the binary and device tree loading code to the machine done
notifier. This allows us to prepare for editing the device tree as part
of the notifier.

This is based on similar code in the ARM virt machine.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220427234146.1130752-2-alistair.francis@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>