This new function will add a single controller of the given model,
except the case of ich9-usb-ehci1 (the master controller for a USB2
controller set) in which case a set of related controllers will be
added (EHCI1, UHCI1, UHCI2, UHCI3). These controllers will not be
given PCI addresses, but should be otherwise ready to use.

"-1" is allowed for controller model, and means "default for this
machinetype". This matches the existing practice in
qemuDomainDefPostParse(), which always adds the default controller
with model = -1, and relies on the commandline builder to set a model
(that is wrong, but will be fixed later).