UefiCpuPkg/VmgExitLib: Add interfaces to set/read GHCB ValidBitmap bits

UefiCpuPkg/VmgExitLib: Add interfaces to set/read GHCB ValidBitmap bits

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3008

In upcoming patches, the setting of the bits in the GHCB ValidBitmap will
be performed in multiple places. In order to reduce code duplication, add
an interface, VmgSetOffsetValid(), to VmgExitLib library to perform this
function. Also, to keep management of the ValidBitmap within the library,
add an inteface, VmgIsOffsetValid(), to return whether the bit in the
ValidBitmap is set for a specified offset.

The new VmgSetOffsetValid() function is a VOID function and will be an
empty function in the VmgExitLibNull implementation of the VmgExitLib
library.

The new VmgIsOffsetValid() function returns a BOOLEAN to indicate if the
offset is valid. This will always return FALSE in the VmgExitLibNull
implementation of the VmgExitLib library.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Acked-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <0bcb2373f8c6e0171ae277d3d7c2eb284621355e.1604685192.git.thomas.lendacky@amd.com>