A new type to track USB addresses.

Every <controller type='usb' index='i'/> is represented by an
object of type virDomainUSBAddressHub located at buses[i].

Each of these hubs has up to 'nports' ports.
If a port is occupied, it has the corresponding bit set in
the 'ports' bitmap, e.g. port 1 would have the 0th bit set.
If there is a hub on this port, then hubs[i] will point
to this hub.