Fix qemu-img convert -O qcow2 -c to NBD URL and add missing test for this
usage.

This already works now, but unfortunately qemu-img fails when trying to
truncate the target image to the same size at the end of the operation.

Changes since v1:
- Include complete code for creating OVA file [Eric]
- Use qcow2 for source file to avoid issues with random CI filesystem [Max]
- Fix many typos [Eric, Max]
- Make qemu_nbd_popen a context manager
- Add more qemu_img_* helpers
- Verify OVA file contents

v1 was here:
https://lists.nongnu.org/archive/html/qemu-block/2020-07/msg01543.html

Nir Soffer (4):
  block: nbd: Fix convert qcow2 compressed to nbd
  iotests: Make qemu_nbd_popen() a contextmanager
  iotests: Add more qemu_img helpers
  iotests: Test convert to qcow2 compressed to NBD

 block/nbd.c                   |  30 ++++++++
 tests/qemu-iotests/264        |  76 ++++++++------------
 tests/qemu-iotests/264.out    |   2 +
 tests/qemu-iotests/302        | 127 ++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/302.out    |  31 +++++++++
 tests/qemu-iotests/group      |   1 +
 tests/qemu-iotests/iotests.py |  34 ++++++++-
 7 files changed, 251 insertions(+), 50 deletions(-)
 create mode 100755 tests/qemu-iotests/302
 create mode 100644 tests/qemu-iotests/302.out

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2.25.4