OvmfPkg/Microvm/fdt: load fdt from fw_cfg

OvmfPkg/Microvm/fdt: load fdt from fw_cfg

Needed for hardware detection: virtio-mmio devices for now,
later also pcie root bridge.

Depends on patched qemu which actually provides an fdt:
https://gitlab.com/kraxel/qemu/-/commits/sirius/microvm-device-tree

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3689
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>