Merge remote-tracking branch 'remotes/gkurz/tags/for-upstream' into staging

Merge remote-tracking branch 'remotes/gkurz/tags/for-upstream' into staging

Just a build fix that I had queued during soft freeze.

# gpg: Signature made Wed 22 Aug 2018 11:22:56 BST
# gpg:                using RSA key 71D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <[email protected]>"
# gpg:                 aka "Gregory Kurz <[email protected]>"
# gpg:                 aka "[jpeg image of size 3330]"
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/for-upstream:
  fsdev: fix compilation with VIRTIO but not VIRTIO_9P

Signed-off-by: Peter Maydell <[email protected]>