RISC-V 64 System Emulator fdt imporperly created after machine init is done
Description of problem:
In commit 49554856 the creation of FDT is moved to `virt_machine_done()`
However, adding guest loader device requires the presence of fdt at `hw/core/guest-loader.c:50` when the fdt ptr is still 0x0.
Thus adding of guest loader device will fail.
Steps to reproduce:
1. Compile Qemu with riscv64 system emulator target
2. Compile Xen hypervisor platform (not necessary)
3. Instruct Qemu start with virt machine and any valid guest-loader device specification.
Additional information:
Commit that changes order of fdt creation: 49554856f0b6f622ab6afdcc275d4ab2ecb3625c