OvmfPkg: supply missing lib class declarations in the DEC file

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>