qemu: Add qemuDomainDefaultUSBControllerModelAutoAdded()

qemu: Add qemuDomainDefaultUSBControllerModelAutoAdded()

In addition to the code in qemuDomainControllerDefPostParse(),
which we have just factored into its own function, we also have
some code in qemuDomainDefAddDefaultDevices() that deals with
choosing the USB controller model for a couple of specific
machine types.

Once again, extract the logic to a dedicated helper. The
behavior is unchanged.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>