OvmfPkg/MemEncryptSevLib: find pages of initial SMRAM save state map

OvmfPkg/MemEncryptSevLib: find pages of initial SMRAM save state map

In the next three patches, we're going to modify three modules under
OvmfPkg. When OVMF is built with -D SMM_REQUIRE and runs in an SEV guest,
each affected module will have to know the page range that covers the
initial (pre-SMBASE relocation) SMRAM save state map. Add a helper
function to MemEncryptSevLib that calculates the "base address" and
"number of pages" constants for this page range.

(In a RELEASE build -- i.e., with assertions disabled and optimization
enabled --, the helper function can be compiled to store two constants
determined at compile time.)

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>