Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20200827' into staging

- document s390x boot devices
- bug fixes

# gpg: Signature made Thu 27 Aug 2020 13:16:41 BST
# gpg:                using RSA key C3D0D66DC3624FF6A8C018CEDECF6B93C6F02FAF
# gpg:                issuer "cohuck@redhat.com"
# gpg: Good signature from "Cornelia Huck <conny@cornelia-huck.de>" [unknown]
# gpg:                 aka "Cornelia Huck <huckc@linux.vnet.ibm.com>" [full]
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>" [full]
# gpg:                 aka "Cornelia Huck <cohuck@kernel.org>" [unknown]
# gpg:                 aka "Cornelia Huck <cohuck@redhat.com>" [unknown]
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/s390x-20200827:
  hw/vfio/ap: Plug memleak in vfio_ap_get_group()
  docs/system/s390x: Add a chapter about s390x boot devices
  virtio-ccw-input: fix description

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