buildd fails to build trusty based images with `mknod: ‘/dev/loopX’: File exists` errors

While trying to build trusty docker images, we are hitting the follow type of error each time (with an apparently random loop device number). We have seen this on every trusty docker build this week. These builds had not been performed since August, when it was successful.

RUN: /usr/share/launchpad-buildd/slavebin/slave-prep
Forking launchpad-buildd slave process...
Kernel version: Linux bos01-arm64-034 4.4.0-97-generic #120-Ubuntu SMP Tue Sep 19 17:31:48 UTC 2017 aarch64
Buildd toolchain package versions: launchpad-buildd_152 python-lpbuildd_152 sbuild_0.67.0-2ubuntu7.1 bzr-builder_0.7.3+bzr174~ppa13~ubuntu14.10.1 bzr_2.7.0-2ubuntu3.1 git-build-recipe_0.3.4~git201611291343.dcee459~ubuntu16.04.1 git_1:2.7.4-0ubuntu1.3 dpkg-dev_1.18.4ubuntu1.2 python-debian_0.1.27ubuntu2.
Syncing the system clock with the buildd NTP service...
12 Oct 16:16:54 ntpdate[1808]: adjust time server 10.211.37.1 offset 0.081996 sec
RUN: /usr/share/launchpad-buildd/slavebin/in-target unpack-chroot --backend=lxd --series=trusty --arch=arm64 LIVEFSBUILD-112474 /home/buildd/filecache-default/76dc409df973c3787fa3d97bbe6fa0d7ccf4bcab
Creating target for build LIVEFSBUILD-112474
RUN: /usr/share/launchpad-buildd/slavebin/in-target mount-chroot --backend=lxd --series=trusty --arch=arm64 LIVEFSBUILD-112474
Starting target for build LIVEFSBUILD-112474
mknod: ‘/dev/loop5’: File exists
Traceback (most recent call last):
  File "/usr/share/launchpad-buildd/slavebin/in-target", line 27, in <module>
    sys.exit(main())
  File "/usr/share/launchpad-buildd/slavebin/in-target", line 23, in main
    return args.operation.run()
  File "/usr/lib/launchpad-buildd/lpbuildd/target/lifecycle.py", line 40, in run
    self.backend.start()
  File "/usr/lib/launchpad-buildd/lpbuildd/target/lxd.py", line 361, in start
    "b", "7", str(minor)])
  File "/usr/lib/launchpad-buildd/lpbuildd/target/lxd.py", line 394, in run
    subprocess.check_call(cmd, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 541, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['lxc', 'exec', 'lp-trusty-arm64', '--', 'linux64', 'mknod', '-m', '0660', '/dev/loop5', 'b', '7', '5']' returned non-zero exit status 1
RUN: /usr/share/launchpad-buildd/slavebin/in-target umount-chroot --backend=lxd --series=trusty --arch=arm64 LIVEFSBUILD-112474
Stopping target for build LIVEFSBUILD-112474
RUN: /usr/share/launchpad-buildd/slavebin/in-target remove-build --backend=lxd --series=trusty --arch=arm64 LIVEFSBUILD-112474
Removing build LIVEFSBUILD-112474

The above log is from https://launchpadlibrarian.net/340643111/buildlog_ubuntu_trusty_arm64_docker-ubuntu-core_BUILDING.txt.gz

Here are some other examples:
https://launchpadlibrarian.net/340326870/buildlog_ubuntu_trusty_armhf_docker-ubuntu-core_BUILDING.txt.gz
https://launchpadlibrarian.net/340326770/buildlog_ubuntu_trusty_amd64_docker-ubuntu-core_BUILDING.txt.gz