compute: add PCI topology and hotplug port reservation docs

[mhenriks]

Document the PCI topology versions (v1, v2, v3), how hotplug port reservation works, and upgrade behavior. Includes troubleshooting guidance for Windows VMs with offline disks after upgrading from KubeVirt 1.6/1.7 to versions with the v3 fix (1.6.4, 1.7.2, 1.8+).

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compute: add PCI topology and hotplug port reservation docs

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[mhenriks]

cc @jean-edouard @vladikr @xpivarc