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

Block layer patches:

- hmp: Fix drive_add ... format=help crash
- block: Forward 'discard' to temporary overlay

# gpg: Signature made Mon 08 Apr 2019 16:43:20 BST
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  hmp: Fix drive_add ... format=help crash
  block: Forward 'discard' to temporary overlay

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