qemu: hotplug: Only label hostdev after checking device conflicts
    
    Similar to what Jiri did for cgroup setup/teardown in 05e149f94, push
    it all into the device handler functions so we can do the necessary prep
    work before claiming the device.
    
    This also fixes hotplugging USB devices by product/vendor (virt-manager's
    default behavior):
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1016511