UefiCpuPkg/MpInitLib: Reuse VMSA allocation to avoid unreserved allocation

https://bugzilla.tianocore.org/show_bug.cgi?id=4353



When parking the APs on exiting from UEFI, a new page allocation is made.
This allocation, however, does not end up being marked reserved in the
memory map supplied to the OS. To avoid this, re-use the VMSA by clearing
the VMSA RMP flag, updating the page contents and re-setting the VMSA RMP
flag.

Fixes: 06544455 ("UefiCpuPkg/MpInitLib: Use SEV-SNP AP Creation ...")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Ray Ni <ray.ni@intel.com>