OvmfPkg: Introduce IoMmuAbsent Protocol GUID

OvmfPkg: Introduce IoMmuAbsent Protocol GUID

Platforms that optionally provide an IOMMU protocol should do so by
including a DXE driver (usually called IoMmuDxe) that produces either
the IOMMU protocol -- if the underlying capabilities are available --,
or gIoMmuAbsentProtocolGuid, to signal that the IOMMU capability
detection completed with negative result (i.e., no IOMMU will be
available in the system).

In turn, DXE drivers (and library instances) that are supposed to use
the IOMMU protocol if it is available should add the following to
their DEPEX:

gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid

This ensures these client modules will only be dispatched after IOMMU
detection completes (with positive or negative result).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leo Duran <leo.duran@amd.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>