usb-bot: hotplug support

This patch marks usb-bot as hot-pluggable device, makes attached
property settable and turns off auto-attach in case the device
was hotplugged.

Hot-plugging a usb-bot device with one or more scsi devices can be
done this way now:

  (1) device-add usb-bot,id=foo
  (2) device-add scsi-{hd,cd},bus=foo.0,lun=0
  (2b) optionally add more devices (luns 0 ... 15).
  (3) qom-set foo.attached = true

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1465984019-28963-5-git-send-email-kraxel@redhat.com