Due to a logic error, the autofilling of USB port when a bus is
specified:
    <address type='usb' bus='0'/>
does not work for non-hub devices on domain startup.

Fix the logic in qemuDomainAssignUSBPortsIterator to also
assign ports for USB addresses that do not yet have one.

https://bugzilla.redhat.com/show_bug.cgi?id=1374128