Commit 815d98a started auto-adding one hub if there are more USB devices
than available USB ports.
This was a strange choice, since there might be even more devices.
Before USB address allocation was implemented in libvirt, QEMU
automatically added a new USB hub if the old one was full.
Adjust the logic to try adding as many hubs as will be needed
to plug in all the specified devices.
https://bugzilla.redhat.com/show_bug.cgi?id=1410188
