OvmfPkg/Microvm/fdt: add empty fdt

OvmfPkg/Microvm/fdt: add empty fdt

FdtClient is unhappy without a device tree, so add an empty fdt
which we can use in case etc/fdt is not present in fw_cfg.

On ARM machines a device tree is mandatory for hardware detection,
that's why FdtClient fails hard.

On microvm the device tree is only used to detect virtio-mmio devices
(this patch series) and the pcie host (future series).  So edk2 can
continue with limited functionality in case no device tree is present:
no storage, no network, but serial console and direct kernel boot
works.

qemu release 6.2 & newer will provide a device tree for microvm.

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>