Merge remote-tracking branch 'remotes/jnsnow/tags/bitmaps-pull-request' into staging

Merge remote-tracking branch 'remotes/jnsnow/tags/bitmaps-pull-request' into staging

# gpg: Signature made Tue 13 Mar 2018 21:11:43 GMT
# gpg:                using RSA key 7DEF8106AAFC390E
# gpg: Good signature from "John Snow (John Huston) <jsnow@redhat.com>"
# Primary key fingerprint: FAEB 9711 A12C F475 812F  18F2 88A9 064D 1835 61EB
#      Subkey fingerprint: F9B7 ABDB BCAC DF95 BE76  CBD0 7DEF 8106 AAFC 390E

* remotes/jnsnow/tags/bitmaps-pull-request:
  iotests: add dirty bitmap postcopy test
  iotests: add dirty bitmap migration test
  migration: add postcopy migration of dirty bitmaps
  migration: allow qmp command migrate-start-postcopy for any postcopy
  migration: add is_active_iterate handler
  migration/qemu-file: add qemu_put_counted_string()
  migration: include migrate_dirty_bitmaps in migrate_postcopy
  qapi: add dirty-bitmaps migration capability
  migration: introduce postcopy-only pending
  dirty-bitmap: add locked state
  block/dirty-bitmap: add _locked version of bdrv_reclaim_dirty_bitmap
  block/dirty-bitmap: fix locking in bdrv_reclaim_dirty_bitmap
  block/dirty-bitmap: add bdrv_dirty_bitmap_enable_successor()

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