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
(cherry picked from commit f06ca25d)