OvmfPkg/VirtioFsDxe: implement EFI_FILE_PROTOCOL.GetPosition, .SetPosition

OvmfPkg/VirtioFsDxe: implement EFI_FILE_PROTOCOL.GetPosition, .SetPosition

Using the functions introduced previously, we can now implement
VirtioFsSimpleFileGetPosition() and VirtioFsSimpleFileSetPosition().

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: <20201216211125.19496-32-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>