bitmaps patches for 2020-09-21

- Eric Blake: Improve 'qemu-img bitmap --merge' by not opening backing images

# gpg: Signature made Tue 22 Sep 2020 01:58:31 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-bitmaps-2020-09-21:
  qemu-img: Support bitmap --merge into backing image

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