ArmVirtPkg: add FDF definition for empty varstore

ArmVirtPkg: add FDF definition for empty varstore

Similar to how OVMF implements this, add a FD definition for the varstore
firmware volume and the FTW areas. The template was taken from the file
OvmfPkg/VarStore.fdf.inc, and subsequently modified to accommodate the
differences in NOR flash layout. This affects the FvLength, Checksum and
BlockMap[0] fields in the FV header, the Size field of the varstore header,
and the Crc and WriteQueueSize fields of the FTW header. The event log
region is not used by ArmVirtQemu, so it has been omitted.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>