OvmfPkg/VmgExitLib: Implement new VmgExitLib interfaces

OvmfPkg/VmgExitLib: Implement new VmgExitLib interfaces

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

The VmgExitLib library added two new interfaces, VmgSetOffsetValid() and
VmgIsOffsetValid(), that must now be implemented in the OvmfPkg version
of the library.

Implement VmgSetOffsetValid() and VmgIsOffsetValid() and update existing
code, that is directly accessing ValidBitmap, to use the new interfaces.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <939e9dc375e6085bc67942fe9a00ecd4c6b77ecf.1604685192.git.thomas.lendacky@amd.com>