virQEMUCapsHasPCIMultiBus: assume true if we have no version information

virQEMUCapsHasPCIMultiBus: assume true if we have no version information

In status XML, we do not store the QEMU version information, we only
format all the capabilities. We dropped QEMU_CAPS_PCI_MULTIBUS
in commit 5b783379 which was released in libvirt 3.2.0.

Therefore the only way of telling if the already running domain
at the time of daemon restart has been started with a QEMU that does
use 'pci.0' or not on PPC is to look at the pci-root controller's
alias. This is not an option if the domain has a user-specified alias
for the pci-root.

Instead of reintroducing the capability, assume 'pci.0' when we have
no version information. That way the only left broken use case would
be the combination of user aliases and very old QEMU.

Partially reverts commit 3a37af1e4.

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