OvmfPkg/MemEncryptSevLib: Address range encryption state interface

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

Update the MemEncryptSevLib library to include an interface that can
report the encryption state on a range of memory. The values will
represent the range as being unencrypted, encrypted, a mix of unencrypted
and encrypted, and error (e.g. ranges that aren't mapped).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <0d98f4d42a2b67310c29bac7bcdcf1eda6835847.1610045305.git.thomas.lendacky@amd.com>

OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf

@@ -36,6 +36,7 @@
 [Sources.X64]
   X64/MemEncryptSevLib.c
   X64/PeiDxeVirtualMemory.c
+  X64/VirtualMemory.c
   X64/VirtualMemory.h
 
 [Sources.IA32]