OvmfPkg/VmgExitLib: Implement library support for VmgExitLib in OVMF

OvmfPkg/VmgExitLib: Implement library support for VmgExitLib in OVMF

The base VmgExitLib library provides a default limited interface. As it
does not provide full support, create an OVMF version of this library to
begin the process of providing full support of SEV-ES within OVMF.

SEV-ES support is only provided for X64 builds, so only OvmfPkgX64.dsc is
updated to make use of the OvmfPkg version of the library.

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>