qemu: prefer to put a Q35 machine's dmi-to-pci-bridge at 00:1E.0

qemu: prefer to put a Q35 machine's dmi-to-pci-bridge at 00:1E.0

This resolves one of the issues listed in:

   https://bugzilla.redhat.com/show_bug.cgi?id=1003983

00:1E.0 is the location of this controller on at least some actual Q35
hardware, so we try to replicate the placement. The bridge should work
just as well in any other location though, so if 00:1E.0 isn't
available, just allow it to be auto-assigned anywhere appropriate.