Cannot start a UEFI vm when pci memory mapped exceed 32GB

[Sergio Rabellino]

Launching a libvirt vm with a UEFI boot with PCI cards (e.g. PCI passthrough of GPU cards) results in a black screen and no boot sequence at all.
This is due to limitations in UEFI Bios boot code (OVMF) which limits to 32GB the max memory mapping, so using GPUs that owns more memory than 32GB (summing up all the GPUs attached) the UEFI boot will fail to start at all.

Following the documentation, latest OVMF has a special flag that can be used to ask to UEFI to allow more than 32GB mem alloc to PCI memory map ( https://edk2.groups.io/g/discuss/topic/59340711 ) which can be specified into the domain XML file for the VM.

The attached patches are a workaround to allow using UEFI with GPUs (tested on a Rocky 9.x with 4 nVIDIA V100 SXM2 cards, with a total of 128GB of pci memory map). The patch tries to sum-up from the host the needed memory aperture and tries to understand if the current OVMF release has the needed options (this should be enhanced further).

HTH