conf: add default USB controller in qemu post-parse callback

conf: add default USB controller in qemu post-parse callback

The parser shouldn't be doing arch-specific things like adding in
implicit controllers to the config. This should instead be done in the
hypervisor's post-parse callback.

This patch removes the auto-add of a usb controller from the domain
parser, and puts it into the qemu driver's post-parse callback (just
as is already done with the auto-add of the pci-root controller). In
the future, any machine/arch that shouldn't have a default usb
controller added should just set addDefaultUSB = false in this
function.

We've recently seen that q35 and ARMV7L domains shouldn't get a default USB
controller, so I've set addDefaultUSB to false for both of those.