MAAS deployments of Eoan (pre-release) failing

[Rod Smith]

MAAS deployments of 19.10 (pre-release) used to work, but are now failing, with the following errors logged (as shown in the MAAS web UI's logs tab):

        Running command ['umount', '/tmp/tmptn44is82/target/dev'] with allowed return codes [0] (capture=False)
        finish: cmd-install/stage-curthooks/builtin/cmd-curthooks: FAIL: curtin command curthooks
        Traceback (most recent call last):
          File "/curtin/curtin/commands/main.py", line 202, in main
            ret = args.func(args)
          File "/curtin/curtin/commands/curthooks.py", line 1442, in curthooks
            builtin_curthooks(cfg, target, state)
          File "/curtin/curtin/commands/curthooks.py", line 1408, in builtin_curthooks
            setup_grub(cfg, target, osfamily=osfamily)
          File "/curtin/curtin/commands/curthooks.py", line 507, in setup_grub
            uefi_remove_old_loaders(grubcfg, target)
          File "/curtin/curtin/commands/curthooks.py", line 344, in uefi_remove_old_loaders
            efi_output = util.get_efibootmgr(target)
          File "/curtin/curtin/util.py", line 875, in get_efibootmgr
            stdout, _ = in_chroot.subp(['efibootmgr', '-v'], capture=True)
          File "/curtin/curtin/util.py", line 689, in subp
            return subp(*args, **kwargs)
          File "/curtin/curtin/util.py", line 275, in subp
            return _subp(*args, **kwargs)
          File "/curtin/curtin/util.py", line 141, in _subp
            cmd=args)
        curtin.util.ProcessExecutionError: Unexpected error while running command.
        Command: ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmptn44is82/target', 'efibootmgr', '-v']
        Exit code: 18
        Reason: -
        Stdout: BootCurrent: 0005
                Timeout: 1 seconds
                BootOrder: 0005,0008,0006,0007,0009,0002
                Boot0002* UEFI: Built-in EFI Shell
         Stderr: Could not parse device path: Invalid argument
                        Unexpected error while running command.
        Command: ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmptn44is82/target', 'efibootmgr', '-v']
        Exit code: 18
        Reason: -
        Stdout: BootCurrent: 0005
                Timeout: 1 seconds
                BootOrder: 0005,0008,0006,0007,0009,0002
                Boot0002* UEFI: Built-in EFI Shell
         Stderr: Could not parse device path: Invalid argument

      Stderr: ''

This happens on several nodes on at least two MAAS servers (both running MAAS 2.6.0-7802-g59416a869-0ubuntu1~18.04.1).

Deployment of earlier versions of Ubuntu is fine; it's only recent Eoan releases that have this problem.