Hi!
The only negative impact it would have is that if someone has old QEMU installed at define time and later upgrades to a QEMU with PCI support before starting the guest, they'll lack the PCI controller.
This won't actually happen, because together with the missing controller he/she will not have PCI devices. Attempt to add a device without a bus immediately produces "No PCI bus" report, and the XML gets rejected. And, right after the upgrade, as soon as he/she adds some PCI devices to use, the XML will be re-parsed, and PCI bus description will be added. Because reparsing happens after any change.
So from that POV, maybe we should just clean up the current merged patch so that it is properly exercised in the test suite by providing capabilities
I will try to do it soon. Kind regards, Pavel Fedin Expert Engineer Samsung Electronics Research center Russia