usb: mtp: tag root property as experimental

Reason: we don't want commit to that interface yet.  Possibly
the implementation will be switched over to use fsdev.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Merge remote-tracking branch 'remotes/kraxel/tags/pull-usb-20140723-1' into staging

usb: mtp: tag root property as experimental

# gpg: Signature made Wed 23 Jul 2014 07:56:21 BST using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-usb-20140723-1:
  usb: mtp: tag root property as experimental

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>