Merge remote-tracking branch 'remotes/kraxel/tags/usb-20180507-pull-request' into...

usb: fixes for mtp and host.

# gpg: Signature made Mon 07 May 2018 10:44:26 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# 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>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/usb-20180507-pull-request:
  usb-host: skip open on pending postload bh
  usb-mtp: Unconditionally check for the readonly bit
  usb-mtp: Add some NULL checks for issues pointed out by coverity

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