OvmfPkg/PlatformPei: correctly align emulated NV storage

Per 2c4b18e ("MdeModulePkg: Add the alignment check for FTW spare area
address and length, and add the check for PcdFlashNvStorageVariableSize
<= PcdFlashNvStorageFtwSpareSize."), FTWDxe refuses to initialize if
spare space base address or size is not aligned to block size.

Depending on configuration, memory for FTWDxe might be dynamically
allocated in PlatformPei. This patch makes sure that the allocated
memory region is aligned.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@14770 6f19259b-4bc3-4df7-8a09-765794883524