ArmVirtPkg/QemuFwCfgLib: move to FDT client protocol

ArmVirtPkg/QemuFwCfgLib: move to FDT client protocol

Make this library depend on the FDT client protocol to access the
host supplied device tree directly rather than depending on VirtFdtDxe
to set them using dynamic PCDs.

Since this library is used by several drivers (BdsDxe, SmbiosPlatformDxe,
SmbiosDxe and QemuFwCfgAcpiPlatformDxe), we will end up parsing the device
tree and the fwcfg node at least four times. However, no dynamic PCDs are
involved anymore, and will even be removed completely in a subsequent
patch. So the conversion is not optimal, but guaranteed to be safe.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>