qemu: prefer the PCI bus alias from status XML

qemu: prefer the PCI bus alias from status XML

For some corner cases, virQEMUCapsHasPCIMultiBus depends on the QEMU
version, which is by design not stored in the status XML and therefore
it cannot be fixed for all existing running domains.

Prefer the controller alias read from the status XML when formatting
PCI addresses and only fall back to using virQEMUCapsHasPCIMultiBus
if the alias is a user alias.

This fixes hotplug after daemon restart for domains not using user
aliases.

Partially reverts commit 937f3195.

https://bugzilla.redhat.com/show_bug.cgi?id=1518148