OvmfPkg/EmuVariableFvbRuntimeDxe: Disable if flash variables are supported

OvmfPkg/EmuVariableFvbRuntimeDxe: Disable if flash variables are supported

If QEMU flash is supported, then the PcdFlashNvStorageVariableBase64
will be set by the flash FVB driver. If it is set to a non-zero value,
then we disable memory based variables.

In future patches we will add the flash FVB driver and
force it to run before this driver. Therefore, if QEMU flash
writing is supported, then this driver will be disabled.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14838 6f19259b-4bc3-4df7-8a09-765794883524