fungi: hi, when I use DIB build diskimage have error, log like
```
2022-04-12 04:58:00.103 | DEBUG diskimage_builder.block_device.utils [-] exec_sudo: /var/cache/nodepool/dib_tmp/dib_build.OuW0QXUa/mnt/: 1.8 GiB (1862995968 bytes) trimmed exec_sudo /usr/lib/python3.6/site-packages/diskimage_builder/block_device/utils.py:135
2022-04-12 04:58:00.106 | INFO diskimage_builder.block_device.utils [-] Calling [sudo umount /var/cache/nodepool/dib_tmp/dib_build.OuW0QXUa/mnt/]
2022-04-12 04:58:00.666 | INFO diskimage_builder.block_device.utils [-] Calling [sudo kpartx -d /dev/loop4]
2022-04-12 04:58:00.713 | DEBUG diskimage_builder.block_device.utils [-] exec_sudo: device-mapper: remove ioctl on loop4p2  failed: Device or resource busy exec_sudo /usr/lib/python3.6/site-packages/diskimage_builder/block_device/utils.py:135
```
I try to run sudo kpartx -d /dev/loop4 manually. it also prompt device or resource busy. when I run reboot, I found /dev/loop4 doesn't exist. I configure diskimage like follow. any help would be appreciated.
```
diskimages:
  - name: ubuntu-focal
    pause: false
    rebuild-age: 172800
    elements:
      - block-device-efi
      - vm
      - simple-init
      - nodepool-base
      - cache-devstack
      - growroot
      - infra-package-needs
      - ubuntu-minimal
    release: focal
    username: zuul
    env-vars:
      TMPDIR: /opt/dib_tmp
      DIB_CHECKSUM: '1'
      DIB_IMAGE_CACHE: /opt/dib_cache
      DIB_APT_LOCAL_CACHE: '0'
      DIB_DISABLE_APT_CLEANUP: '0'
      DIB_DEBOOTSTRAP_EXTRA_ARGS: '--no-check-gpg'
      DIB_DEBIAN_COMPONENTS: 'main,universe'
      DIB_CUSTOM_PROJECTS_LIST_URL: 'file:///etc/nodepool/repos.yaml'
      GIT_HTTP_LOW_SPEED_TIME: '600'
      GIT_HTTP_LOW_SPEED_LIMIT: '1000'
```