Platform/ARM/SgiPkg: Restructure virtio device registration

SGI platforms support multiple virtio devices. So the existing code, that
supports registration of only the virtio disk, is restructured to
accommodate the registration of additional virtio devices.

In addition to this, PCDs to represent the virtio controller base and
address space size are introduced.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Daniil Egranov <daniil.egranov@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>