OvmfPkg/VirtioFsDxe: DriverBinding: open VirtioDevice, install SimpleFs

Complete the Supported, Start, and Stop member functions of
EFI_DRIVER_BINDING_PROTOCOL sufficiently for exercising the UEFI driver
model:

- bind virtio-fs devices,

- produce placeholder EFI_SIMPLE_FILE_SYSTEM_PROTOCOL instances on them.

On the "TO_START" (= Virtio) side, the VirtioFsBindingSupported() function
verifies the Virtio subsystem ID for the virtio-fs device (decimal 26 --
see
<https://github.com/oasis-tcs/virtio-spec/blob/87fa6b5d8155/virtio-fs.tex>).
Beyond that, no actual Virtio setup is performed for now. Those bits are
going to be implemented later in this series.

On the "BY_START" (= UEFI filesystem) side, the VirtioFsOpenVolume()
function -- which is the sole EFI_SIMPLE_FILE_SYSTEM_PROTOCOL member
function -- is a stub; it always returns EFI_NO_MEDIA, for now.

The "CONNECT", "DISCONNECT", and "MAP -R" UEFI Shell commands can be used
to test this patch.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201216211125.19496-4-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/VirtioFsDxe/SimpleFsOpenVolume.c

@@ -0,0 +1,26 @@
+/** @file
+  EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.OpenVolume() member function for the Virtio
+  Filesystem driver.
+
+  Copyright (C) 2020, Red Hat, Inc.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include "VirtioFsDxe.h"
+
+/**
+  Open the root directory on the Virtio Filesystem.
+
+  Refer to EFI_SIMPLE_FILE_SYSTEM_PROTOCOL_OPEN_VOLUME for the interface
+  contract.
+**/
+EFI_STATUS
+EFIAPI
+VirtioFsOpenVolume (
+  IN  EFI_SIMPLE_FILE_SYSTEM_PROTOCOL *This,
+  OUT EFI_FILE_PROTOCOL               **Root
+  )
+{
+  return EFI_NO_MEDIA;
+}