Merge remote-tracking branch 'remotes/borntraeger/tags/s390x-20190923' into staging

- bugfixes in ccw bios
- gen15a is called z15
- officially require a 3.15 kernel or later for kvm

# gpg: Signature made Mon 23 Sep 2019 08:18:32 BST
# gpg:                using RSA key 117BBC80B5A61C7C
# gpg: Good signature from "Christian Borntraeger (IBM) <borntraeger@de.ibm.com>" [full]
# Primary key fingerprint: F922 9381 A334 08F9 DBAB  FBCA 117B BC80 B5A6 1C7C

* remotes/borntraeger/tags/s390x-20190923:
  s390x/cpumodel: Add the z15 name to the description of gen15a
  s390x/kvm: Officially require at least kernel 3.15
  pc-bios/s390-ccw: Rebuild the s390-netboot.img firmware image
  pc-bios/s390-ccw/net: fix a possible memory leak in get_uuid()
  pc-bios/s390-ccw: Do not pre-initialize empty array

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