libxl: add HVM usb passthrough support

libxl: add HVM usb passthrough support

Add HVM usb passthrough support to libxl by using qemu's capability
to emulate standard USB controllers.

A USB controller is added via qmp command to the emulated hardware
when a usbctrl device of type DEVICEMODEL is requested. Depending on
the requested speed the appropriate hardware type is selected. A host
USB device can then be added to the emulated USB controller via qmp
command.

Removing of the devices is done via qmp commands, too.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>