OvmfPkg/QemuFlashFvbServices: factor out SetPcdFlashNvStorageBaseAddresses

OvmfPkg/QemuFlashFvbServices: factor out SetPcdFlashNvStorageBaseAddresses

Extract the dynamic setting of the
- PcdFlashNvStorageVariableBase64
- PcdFlashNvStorageFtwWorkingBase
- PcdFlashNvStorageFtwSpareBase
addresses to a helper function.

For now, the helper function is identical (duplicated) between the SMM
flash driver and the runtime DXE flash driver. In subsequent patches, this
will change.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=386
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200310222739.26717-3-lersek@redhat.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>