hw/openrisc: Support monitor dumpdtb command

hw/openrisc: Support monitor dumpdtb command

The openrisc machines don't set MachineState::fdt to point to their
DTB blob.  This means that although the command line '-machine
dumpdtb=file.dtb' option works, the equivalent QMP and HMP monitor
commands do not, but instead produce the error "This machine doesn't
have a FDT".

Set MachineState::fdt in openrisc_load_fdt(), when we write it to
guest memory.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250206151214.2947842-3-peter.maydell@linaro.org