Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2019-05-07' into staging

Block patches:
- Fixes to qcow2's implementation of qemu-img check
- Our SSH driver now supports bdrv_refresh_filename()
- Miscellaneous fixes

# gpg: Signature made Tue 07 May 2019 16:16:58 BST
# gpg:                using RSA key F407DB0061D5CF40
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>" [full]
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2019-05-07:
  iotests: Fix iotests 110 and 126
  commit: Use bdrv_append() in commit_start()
  block: Assert that drv->bdrv_child_perm is set in bdrv_child_perm()
  block/ssh: Implement .bdrv_dirname()
  block/ssh: Implement .bdrv_refresh_filename()
  qcow2: discard bitmap when removed
  qcow2-refcount: don't mask corruptions under internal errors
  qcow2-refcount: check_refcounts_l2: don't count fixed cluster as allocated
  qcow2-refcount: check_refcounts_l2: reduce ignored overlaps
  qcow2-refcount: avoid eating RAM
  qcow2-refcount: fix check_oflag_copied

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