Upgrade fails moving syslog config files "invalid argument"

The fix for bug 1464335 which changed the rsyslog upgrade steps breaks upgrade.

<http://reports.vapour.ws/releases/2814/job/aws-upgrade-trusty-amd64/attempt/2165>

machines:
  "0":
    agent-state: error
    agent-state-info: |-
      upgrade to 1.24.2 failed (giving up): move syslog config from LogDir to DataDir: error(s) while moving old syslog config files: invalid argument
      invalid argument
      invalid argument
      invalid argument
      invalid argument
    agent-version: 1.24.2

From machine-0.log:

2015-06-25 03:35:40 INFO juju.upgrade upgrade.go:133 running upgrade step: move syslog config from LogDir to DataDir
2015-06-25 03:35:40 DEBUG juju.upgrade steps124.go:98 Copying "/var/log/juju/ca-cert.pem" to "/etc/juju/ca-cert.pem"
2015-06-25 03:35:40 DEBUG juju.upgrade steps124.go:98 Copying "/var/log/juju/rsyslog-cert.pem" to "/etc/juju/rsyslog-cert.pem"
2015-06-25 03:35:40 DEBUG juju.upgrade steps124.go:98 Copying "/var/log/juju/rsyslog-key.pem" to "/etc/juju/rsyslog-key.pem"
2015-06-25 03:35:40 DEBUG juju.upgrade steps124.go:98 Copying "/var/log/juju/logrotate.conf" to "/etc/juju/logrotate.conf"
2015-06-25 03:35:40 DEBUG juju.upgrade steps124.go:98 Copying "/var/log/juju/logrotate.run" to "/etc/juju/logrotate.run"
2015-06-25 03:35:40 ERROR juju.upgrade upgrade.go:135 upgrade step "move syslog config from LogDir to DataDir" failed: error(s) while moving old syslog config files: invalid argument
invalid argument
invalid argument
invalid argument
invalid argument
2015-06-25 03:35:40 ERROR juju.cmd.jujud upgrade.go:359 upgrade from 1.23.3 to 1.24.2 for "machine-0" failed (will retry): move syslog config from LogDir to DataDir: error(s) while moving old syslog config files: invalid argument
invalid argument
invalid argument
invalid argument
invalid argument

Bad revision is 70d9e74126b93ce4c837e277a2dd96fd1674bf7f:

<http://reviews.vapour.ws/r/1963/>
<https://github.com/juju/juju/pull/2591>
<>