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

Some 9pfs bugs fixes: potential hang at reset, migration blocker leak.

# gpg: Signature made Tue 04 Apr 2017 17:07:55 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@linux.vnet.ibm.com>"
# gpg:                 aka "Gregory Kurz (Groug) <groug@free.fr>"
# gpg:                 aka "[jpeg image of size 3330]"
# 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: clear migration blocker at session reset
  9pfs: fix multiple flush for same request

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