OvmfPkg: introduce VirtioFsDxe

OvmfPkg: introduce VirtioFsDxe

The purpose of the driver is to ease file exchange (file sharing) between
the guest firmware and the virtualization host. The driver is supposed to
interoperate with QEMU's "virtiofsd" (Virtio Filesystem Daemon).

References:
- https://virtio-fs.gitlab.io/
- https://libvirt.org/kbase/virtiofs.html

VirtioFsDxe will bind virtio-fs devices, and produce
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL instances on them.

In the longer term, assuming QEMU will create "bootorder" fw_cfg file
entries for virtio-fs devices, booting guest OSes from host-side
directories should become possible (dependent on the matching
QemuBootOrderLib enhancement).

Add the skeleton of the driver. Install EFI_DRIVER_BINDING_PROTOCOL with
stub member functions. Install EFI_COMPONENT_NAME2_PROTOCOL with final
member functions. This suffices for the DRIVERS command in the UEFI Shell
to list the driver with a human-readable name.

The file permission model is described immediately in the INF file as a
comment block, for future reference.

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: <20201216211125.19496-2-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>