OvmfPkg/VirtioFsDxe: implement EFI_FILE_PROTOCOL.Open()
    
    Using the functions introduced previously, we can now implement
    VirtioFsSimpleFileOpen().
    
    This lets the "MKDIR" command to work in the UEFI shell, for example.
    
    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: <20201216211125.19496-27-lersek@redhat.com>
    Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>