Some tools, such as virt-manager, prefers having the default USB
controller explicit in the XML document. This patch makes sure there
is one. With this patch, it is now possible to switch from USB1 to
USB2 from the release 0.9.1 of virt-manager.
Fix tests to pass with this change.
Prefer bootindex=N option for -device over the old way -boot ORDER
possibly accompanied with boot=on option for -drive. This gives us full
control over which device will actually be used for booting guest OS.
Moreover, if qemu doesn't support boot=on, this is the only way to boot
of certain disks in some configurations (such as virtio disks when used
together IDE disks) without transforming domain XML to use per device
boot elements.
