Recreate the USB address cache at reconnect
    
    When starting a new domain, we allocate the USB addresses and keep
    an address cache in the domain object's private data.
    
    However this data is lost on libvirtd restart.
    
    Also generate the address cache if all the addresses have been
    specified, so that devices hotplugged after libvirtd restart
    also get theirs assigned.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1387666