Merge remote-tracking branch 'remotes/armbru/tags/pull-misc-2019-04-02' into staging

Merge remote-tracking branch 'remotes/armbru/tags/pull-misc-2019-04-02' into staging

Miscellaneous patches for 2019-04-02

# gpg: Signature made Tue 02 Apr 2019 12:54:27 BST
# gpg:                using RSA key 3870B400EB918653
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-misc-2019-04-02:
  accel: Unbreak accelerator fallback
  vl: Document dependencies hiding in global and compat props
  migration: Support adding migration blockers earlier
  Revert "migration: move only_migratable to MigrationState"
  Revert "vl: Fix to create migration object before block backends again"
  qapi/migration.json: Rename COLOStatus last_mode to last-mode
  qapi/migration.json: Fix ColoStatus member last_mode's version
  vl: Fix error location of positional arguments

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