OvmfPkg: supply missing lib class declarations in the DEC file
    
    List the header files in the OvmfPkg DEC file for the following lib
    classes:
    
    - MemEncryptSevLib (one instance: BaseMemEncryptSevLib)
    
    - PlatformFvbLib (two instances: EmuVariableFvbLib, PlatformFvbLibNull)
    
    - VirtioLib (one instance: VirtioLib)
    
    - VirtioMmioDeviceLib (one instance: VirtioMmioDeviceLib)
    
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Cc: Sean Brogan <sean.brogan@microsoft.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2662
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Message-Id: <20200407100545.25406-1-lersek@redhat.com>
    Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>