Introduce functions in libxl to add and remove USB devices for an PV guest

    Signed-off-by: Simon Cao <caobosimon@gmail.com>
        ---
        CC: George Dunlap <george.dunlap@eu.citrix.com>
        CC: Ian Jackson <ian.jackson@citrix.com>
        CC: Ian Campbell <ian.campbell@citrix.com>
        CC: Pasi Kärkkäinen <pasik@iki.fi>
        CC: Lars Kurth <lars.kurth@citrix.com>
xl: Add commands for usb hot-plug

    Signed-off-by: Simon Cao <caobosimon@gmail.com>
            ---
            CC: George Dunlap <george.dunlap@eu.citrix.com>
            CC: Ian Jackson <ian.jackson@citrix.com>
            CC: Ian Campbell <ian.campbell@citrix.com>
            CC: Pasi Kärkkäinen <pasik@iki.fi>
            CC: Lars Kurth <lars.kurth@citrix.com>
changes to be made
Merge branch 'out/xlForPVUSB' of github.com:caobosco/libxlPVUSB into out/xlForPVUSB

Conflicts:
	tools/libxl/libxl_usb.c