OvmfPkg: consolidate variable driver stack in DSC and FDF files

OvmfPkg: consolidate variable driver stack in DSC and FDF files

The following modules constitute the variable driver stack:

- QemuFlashFvbServicesRuntimeDxe and EmuVariableFvbRuntimeDxe, runtime
  alternatives for providing the Firmware Volume Block(2) Protocol,
  dependent on qemu pflash presence,

- FaultTolerantWriteDxe, providing the Fault Tolerant Write Protocol,

- MdeModulePkg/Universal/Variable/RuntimeDxe, independently of
  -D SECURE_BOOT_ENABLE, providing the Variable and Variable Write
  Architectural Protocols.

Let's move these drivers closer to each other in the DSC and FDF files, so
that we can switch the variable driver stack to SMM with more local
changes.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19064 6f19259b-4bc3-4df7-8a09-765794883524