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>

OvmfPkg/OvmfPkg.dec

@@ -22,6 +22,10 @@
   #
   LoadLinuxLib|Include/Library/LoadLinuxLib.h
 
+  ##  @libraryclass  Declares helper functions for Secure Encrypted
+  #                  Virtualization (SEV) guests.
+  MemEncryptSevLib|Include/Library/MemEncryptSevLib.h
+
   ##  @libraryclass  Save and restore variables using a file
   #
   NvVarsFileLib|Include/Library/NvVarsFileLib.h
@@ -45,6 +49,9 @@
   #                  return codes, to the UEFI console.
   PlatformBmPrintScLib|Include/Library/PlatformBmPrintScLib.h
 
+  ##  @libraryclass  Customize FVB2 protocol member functions for a platform.
+  PlatformFvbLib|Include/Library/PlatformFvbLib.h
+
   ##  @libraryclass  Access QEMU's firmware configuration interface
   #
   QemuFwCfgLib|Include/Library/QemuFwCfgLib.h
@@ -67,6 +74,13 @@
   #
   SerializeVariablesLib|Include/Library/SerializeVariablesLib.h
 
+  ##  @libraryclass  Declares utility functions for virtio device drivers.
+  VirtioLib|Include/Library/VirtioLib.h
+
+  ##  @libraryclass  Install Virtio Device Protocol instances on virtio-mmio
+  #                  transports.
+  VirtioMmioDeviceLib|Include/Library/VirtioMmioDeviceLib.h
+
   ##  @libraryclass  Invoke Xen hypercalls
   #
   XenHypercallLib|Include/Library/XenHypercallLib.h