OvmfPkg: Add NV Variable storage within FD

OvmfPkg: Add NV Variable storage within FD

This is to prepare for QEMU flash support which will allow
non-volatile variables to be saved in the flash image.

Note two size changes:
 * NV Varstore size increased from 0xc000 to 0xe000
 * FTW work size decreased from 0x2000 to 0x1000

The reason for this change is that the fault-tolerant write
support requires that the work area fit within the block
just before the fault-tolerant write spare storage blocks.
Since QEMU flash blocks have a size of 0x1000, this means
that the maximum FTW work size is 0x1000.

v2:
 * Update commit message and PcdVariableStoreSize

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@14835 6f19259b-4bc3-4df7-8a09-765794883524