usb: make attach optional.

usb: make attach optional.

Add a auto_attach field to USBDevice, which is enabled by default.
USB drivers can clear this field in case they do *not* want the device
being attached (i.e. plugged into a usb port) automatically after
successfull init().

Use cases (see next patches):
 * attaching encrypted mass storage devices.
 * -usbdevice host:...

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>