OvmfPkg: Add IoMmuDxe driver

OvmfPkg: Add IoMmuDxe driver

The IOMMU protocol driver provides capabilities to set a DMA access
attribute and methods to allocate, free, map and unmap the DMA memory
for the PCI Bus devices.

Due to security reasons all DMA operations inside the SEV guest must
be performed on shared (i.e unencrypted) pages. The IOMMU protocol
driver for the SEV guest uses a bounce buffer to map guest DMA buffer
to shared pages inorder to provide the support for DMA operations inside
SEV guest.

IoMmuDxe driver looks for SEV capabilities, if present then it installs
the real IOMMU protocol otherwise it installs placeholder protocol.
Currently, PciHostBridgeDxe and QemuFWCfgLib need to know the existance
of IOMMU protocol. The modules needing to know the existance of IOMMU
support should add

  gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid

in their depex to ensure that platform IOMMU detection has been performed.

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