ArmVirtPkg/VirtFdtDxe: remove Xenio handling and rename to VirtioFdtDxe

ArmVirtPkg/VirtFdtDxe: remove Xenio handling and rename to VirtioFdtDxe

Now that we have moved the handling of the xen,xen DT node to XenioFdtDxe,
remove its handling from VirtFdtDxe. Since the only functionality that
remains is handling the virtio,mmio DT node, rename VirtFdtDxe to
VirtioFdtDxe to reflect that. Also update the platforms that use this
driver.

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