Description of problem:

For certain UTF8 string lengths systemd-run fails to run a command in a separate machine

Version-Release number of selected component (if applicable):

- CentOS 7.4
- systemd-219-42.el7_4.4.x86_64
- Linux vm5.dpnet.nethesis.it 3.10.0-693.11.6.el7.x86_64 #1 SMP Thu Jan 4 01:06:37 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

How reproducible:

always

Steps to Reproduce:

1. prepare the container chroot:
   yum -y --releasever=/ --installroot=/var/lib/machines/c1 install bash systemd

2. spawn a container:
   /usr/bin/systemd-nspawn --boot --machine=c1 -D /var/lib/machines/c1

3. open a new terminal and run the script:
 
   for ((I=1; I<10; I++)); do USERNAME=$(printf 'à%.0s' `seq 1 $I`); systemd-run -M c1 -q -t /usr/bin/echo $I $USERNAME; done

Actual results:

1 à
2 àà
3 ààà
4 àààà
5 ààààà
Failed to start transient service unit: Message did not receive a reply (timeout by message bus)

Expected results (with "a" instead of "à"):

1 a
2 aa
3 aaa
4 aaaa
5 aaaaa
6 aaaaaa
7 aaaaaaa
8 aaaaaaaa
9 aaaaaaaaa

Additional info:

LANG=en_US.UTF-8
Running in KVM
This bug was originally reported here:

https://github.com/NethServer/dev/issues/5400