OvmfPkg/Sec: Build identity mapped pages in RAM for X64

OvmfPkg/Sec: Build identity mapped pages in RAM for X64

This is based on MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.c.

Previously we would run using page tables built into the
firmware device.

If a flash memory is available, it is unsafe for the page
tables to be stored in memory since the processor may try
to write to the page table data structures.

Additionally, when KVM ROM support is enabled for the
firmware device, then PEI fails to boot when the page
tables are in the firmware device.

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@14494 6f19259b-4bc3-4df7-8a09-765794883524