When support for dmi-to-pci-bridge was added, it was assumed that,
just as with the pci-root bus, slot 0 was reserved. This is not the
case - it can be used to connect a device just like any other slot, so
remove the restriction and update the test cases that auto-assign an
address on a dmi-to-pci-bridge.
Some places already check for "virt-" prefix as well as plain "virt".
virQEMUCapsHasPCIMultiBus did not, resulting in multiple PCI devices
having assigned the same unnumbered "pci" alias.
Add a test for the "virt-2.6" machine type which also omits the
<model type='virtio'/> in <interface>, to check if
qemuDomainDefaultNetModel works too.
https://bugzilla.redhat.com/show_bug.cgi?id=1325085
