OvmfPkg/BaseMemEncryptSevLib: Enable protection for newly added page table

OvmfPkg/BaseMemEncryptSevLib: Enable protection for newly added page table

Commit 2ac1730bf2a5 (MdeModulePkg/DxeIpl: Mark page table as read-only)
sets the memory pages used for page table as read-only after paging is
setup and sets CR0.WP to protect CPU modifying the read-only pages.
The commit causes #PF when MemEncryptSevClearPageEncMask() or
MemEncryptSevSetPageEncMask() tries to change the page-table attributes.

This patch takes the similar approach as Commit 147fd35c3e38
(UefiCpuPkg/CpuDxe: Enable protection for newly added page table).
When page table protection is enabled, we disable it temporarily before
changing the page table attributes.

This patch makes use of the same approach as Commit 2ac1730bf2a5
(MdeModulePkg/DxeIpl: Mark page table as read-only)) for allocating
page table memory from reserved memory pool, which helps to reduce a
potential "split" operation.

The patch duplicates code from commit 147fd35c3e38. The code duplication
will be removed after we implement page table manipulation library. See
bugzilla https://bugzilla.tianocore.org/show_bug.cgi?id=847.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>