OvmfPkg/VirtioInputDxe: implement key notifications

Presently, VirtioInputDxe accepts key notifications (RegisterKeyNotifyEx)
but does not act upon them. Despite ConSplitter correctly registering
notifications via all input devices (including PS/2 etc. devices
that implement key notifications correctly), emulators such as QEMU
stop sending input to non-virtio keyboards as soon as the virtio device
is claimed. As a result, preboot hotkeys are non-functional when virtio
is used.

Fixes: https://github.com/tianocore/edk2/issues/12887

Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>
Tested-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>