usb: Add USB3 capabilities to Spice USB devices

usb: Add USB3 capabilities to Spice USB devices

To not change current behaviour and thus breaking live migration USB3
for a Spice USB device requires Qemu v4.1.

The old behavior was that even though technically it was possible to
the set `usb3=1` setting, it was ignored. The bus was hardcoded to
ehci. If another USB2 device was added or the machine type was set to
Q35 an ehci controller was present and the VM was able to boot.

With this patch the behaviour is changing and the bus is set to xhci if
USB3 is set for the Spice USB device and the VM is running under Qemu
v4.1.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>