Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20170320' into staging

One bugfix for device plug/unplug and migration in the
channel subsystem code.

# gpg: Signature made Mon 20 Mar 2017 08:45:59 GMT
# gpg:                using RSA key 0xDECF6B93C6F02FAF
# gpg: Good signature from "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/s390x-20170320:
  s390x/css: reassign subchannel if schid is changed after migration

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