usb: replace handle_destroy with unrealize

Curiously, unrealize() is not being used, but it seems more
appropriate than handle_destroy() together with realize(). It is more
ubiquitous destroy name in qemu code base and may throw errors.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170221141451.28305-25-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
