OvmfPkg/IoMmuDxe: unmap all IOMMU mappings at ExitBootServices()

OvmfPkg/IoMmuDxe: unmap all IOMMU mappings at ExitBootServices()

Register an ExitBootServices() callback that tears down all IOMMU
mappings, without modifying the UEFI memory map.

The trick is that in the ExitBootServices() callback, we don't immediately
do the work; instead we signal another (private) event.

Normally the dispatch order of ExitBootServices() callbacks is unspecified
(within the same task priority level anyway). By queueing another
function, we delay the unmapping until after all PciIo and Virtio drivers
abort -- in their own ExitBootServices() callbacks -- the pending DMA
operations of their respective controllers.

Furthermore, the fact that IoMmuUnmapWorker() rewrites client-owned memory
when it unmaps a Write or CommonBuffer bus master operation, is safe even
in this context. The existence of any given "MapInfo" in "mMapInfos"
implies that the client buffer pointed-to by "MapInfo->CryptedAddress" was
live when ExitBootServices() was entered. And, after entering
ExitBootServices(), nothing must have changed the UEFI memory map, hence
the client buffer at "MapInfo->CryptedAddress" still exists.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>