ArmVirtPkg/ArmVirtXen: move from VirtFdtDxe to new XenioFdtDxe driver

ArmVirtPkg/ArmVirtXen: move from VirtFdtDxe to new XenioFdtDxe driver

Now that the only functionality that remains in VirtFdtDxe is enumerating
the respective virtual I/O buses, it no longer makes sense to have a driver
that is shared between Xen domU and QEMU. So move the Xen I/O DT node
handling to a new driver, and update ArmVirtXen to switch to it.

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