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

This pull request contains:
- a fix for a regression introduced in 2.7
- basic functional testing for virtio-9p
- some code cleanups for 9pfs

# gpg: Signature made Mon 19 Sep 2016 10:40:17 BST
# gpg:                using DSA key 0x02FC3AEB0101DBC2
# gpg: Good signature from "Greg Kurz <groug@kaod.org>"
# gpg:                 aka "Greg Kurz <groug@free.fr>"
# gpg:                 aka "Greg Kurz <gkurz@fr.ibm.com>"
# gpg:                 aka "Greg Kurz <gkurz@linux.vnet.ibm.com>"
# gpg:                 aka "Gregory Kurz (Groug) <groug@free.fr>"
# gpg:                 aka "Gregory Kurz (Cimai Technology) <gkurz@cimai.com>"
# gpg:                 aka "Gregory Kurz (Meiosys Technology) <gkurz@meiosys.com>"
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 2BD4 3B44 535E C0A7 9894  DBA2 02FC 3AEB 0101 DBC2

* remotes/gkurz/tags/for-upstream:
  9pfs: fix potential segfault during walk
  tests: virtio-9p: add basic configuration test
  tests: virtio-9p: introduce start/stop functions
  9pfs: introduce v9fs_path_sprintf() helper
  9pfs: drop useless v9fs_string_null() function
  9pfs: drop duplicate line in proxy backend
  9pfs: drop unused fmt strings in the proxy backend

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