The behavior of SPICE USB redirection has changed when redirection is disabled. From the dev bug 1373223: If SPICE USB redirection is enabled (VM-like dialog > Console > USB Support), the behavior remains unchanged: each VM has a quadruple of usb controllers: ich9-ehci1, ich9-uhci1,ich9-uhci2, ich9-uhci3. If SPICE USB redirection is disabled, then VM has newly USB controller as specified in osinfo-defaults.properties configuration file, i.e. it is configurable per guest operating system and effective cluster version. Previously no usb controller was send to libvirt and libvirt created a default usb controller. Default for all intel (x86, x86-64) operating systems is "piix3-uhci", for ppc64 systems it is "nec-xhci". SME feedback from Jakub Niedermertl about the 'USB Support' field in the 'Console' tab of the 'New Virtual Machine' window: Despite the name this dropdown controls spice usb redirection. Enable: spice usb redirection is enabled allowing client to attach his/her usb device to a guest VM. VM is added special set of special 4 usb controllers. Disabled: usb spice redirection is disabled, USB controller devices are added according to "devices.usb.controller" key in "osinfo-defaults.properties" configuration file.
Moving to 4.2.
Published document: https://access.redhat.com/documentation/en-us/red_hat_virtualization/4.2/html-single/virtual_machine_management_guide/index#Virtual_Machine_Console_settings_explained