refactor: add pdev fallback

[ejose19]

This PR intends to extend patch-0001-libxl-conditionally-allow-PCI-passthrough-on-PV-with.patch in order to allow using pci_get_pdev when pci_get_pdev_by_domain was unsuccessful, as it seems that update broke something that incorrectly reports the owner of a device thus rendering pci_get_pdev_by_domain useless (see QubesOS/qubes-issues#3217 (comment))

I see the base patch already includes some utilities to make this work, but I would like to ask before proceed:

• should we import libxl__is_insecure_pv_passthrough_enabled or redefine in this file?
• should iommu_enabled be stored globally so it's easier to get (as it will probably be used in more patches)
• is type == LIBXL_DOMAIN_TYPE_PV check redundant in this exact location?

I'm not very knowledgeable in C but given some advise regarding these questions I can complete this PR. Also if you see that this PR should be another flag and not be used implicitly with qubes.enable_insecure_pv_passthrough, please let me know.

[marmarek]

it seems that update broke something that incorrectly reports the owner of a device thus rendering pci_get_pdev_by_domain useless

I'd first like to find one why pci_get_pdev_by_domain fails. Only then it will be possible to find a solution (which may be either fallback to pci_get_pdev, or changes elsewhere to make pci_get_pdev_by_domain return the device).