OvmfPkg/VmgExitLib: Add support for VMMCALL NAE events

OvmfPkg/VmgExitLib: Add support for VMMCALL NAE events

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Under SEV-ES, a VMMCALL intercept generates a #VC exception. VMGEXIT must
be used to allow the hypervisor to handle this intercept.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>