Description of problem:

When something goes wrong with the CreateMachine DBus call though all libvirt ever seems to get back is  "Input/output error".

After strace'ing systemd-machined we find the real error

recvmsg(5, {msg_name(0)=NULL, msg_iov(1)=[{"l\1\0\1\334\0\0\0\2\0\0\0\277\0\0\0\1\1o\0\31\0\0\0/org/freedesktop/machine1\0\0\0\0\0\0\0\6\1s\0\30\0\0\0org.freedesktop.machine1\0\0\0\0\0\0\0\0\2\1s\0 \0\0\0org.freedesktop.machine1.Manager\0\0\0\0\0\0\0\0\3\1s\0\r\0\0\0CreateMachine\0\0\0\10\1g\0\fsayssusa(sv)\0\0\0\0\0\0\0\7\1s\0\6\0\0\0:1.130\0\0\t\0\0\0lxc-busy2\0\0\0\20\0\0\0\335\247\271G\10F\27Y(s\0177]\367\327\353\v\0\0\0libvirt-lxc\0\t\0\0\0container\0\0\0\210:\0\0\0\0\0\0\0\0\0\0\204\0\0\0\0\0\0\0\5\0\0\0Slice\0\1s\0\0\0\0\16\0\0\0/machine.slice\0\0\0\0\0\0\r\0\0\0CPUAccounting\0\1b\0\0\0\0\1\0\0\0\0\0\0\0\21\0\0\0BlockIOAccounting\0\1b\0\0\0\0\1\0\0\0\20\0\0\0MemoryAccounting\0\1b\0\1\0\0\0", 2048}], msg_controllen=0, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 428
sendmsg(5, {msg_name(0)=NULL, msg_iov(2)=[{"l\1\0\1D\1\0\0\n\0\0\0\255\0\0\0\1\1o\0\31\0\0\0/org/freedesktop/systemd1\0\0\0\0\0\0\0\6\1s\0\30\0\0\0org.freedesktop.systemd1\0\0\0\0\0\0\0\0\2\1s\0 \0\0\0org.freedesktop.systemd1.Manager\0\0\0\0\0\0\0\0\3\1s\0\22\0\0\0StartTransientUnit\0\0\0\0\0\0\10\1g\0\7ssa(sv)\0\0\0\0", 192}, {"\32\0\0\0machine-lxc\\x2dbusy2.scope\0\0\4\0\0\0fail\0\0\0\0\24\1\0\0\5\0\0\0Slice\0\1s\0\0\0\0\r\0\0\0machine.slice\0\0\0\0\0\0\0\v\0\0\0Description\0\1s\0\0\23\0\0\0Container lxc-busy2\0\0\0\0\0\17\0\0\0TimeoutStopUSec\0\1t\0\0 \241\7\0\0\0\0\0\4\0\0\0PIDs\0\2au\0\0\0\0\4\0\0\0\210:\0\0\5\0\0\0Slice\0\1s\0\0\0\0\16\0\0\0/machine.slice\0\0\0\0\0\0\r\0\0\0CPUAccounting\0\1b\0\0\0\0\1\0\0\0\0\0\0\0\21\0\0\0BlockIOAccounting\0\1b\0\0\0\0\1\0\0\0\20\0\0\0MemoryAccounting\0\1b\0\1\0\0\0", 324}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 516
recvmsg(5, {msg_name(0)=NULL, msg_iov(1)=[{"l\3\1\1+\0\0\0\265\1\0\0]\0\0\0\6\1s\0\6\0\0\0:1.126\0\0\4\1s\0$\0\0\0org.freedesktop.systemd1.InvalidName\0\0\0\0\5\1u\0\n\0\0\0\10\1g\0\1s\0\0\7\1s\0\4\0\0\0:1.1\0\0\0\0&\0\0\0Unit name /machine.slice is not valid.\0", 2048}], msg_controllen=0, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 155
sendmsg(3, {msg_name(0)=NULL, msg_iov(4)=[{"PRIORITY=3\nSYSLOG_FACILITY=4\nCODE_FILE=src/machine/machine.c\nCODE_LINE=246\nCODE_FUNCTION=machine_start_scope\nSYSLOG_IDENTIFIER=systemd-machined\n", 144}, {"MESSAGE=", 8}, {"Failed to start machine scope: Unit name /machine.slice is not valid.", 69}, {"\n", 1}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 222
sendmsg(5, {msg_name(0)=NULL, msg_iov(2)=[{"l\3\1\1\27\0\0\0\v\0\0\0O\0\0\0\6\1s\0\6\0\0\0:1.130\0\0\4\1s\0\"\0\0\0org.freedesktop.DBus.Error.IOError\0\0\0\0\0\0\5\1u\0\2\0\0\0\10\1g\0\1s\0\0", 96}, {"\22\0\0\0Input/output error\0", 23}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 119


So machined is getting a useful error back from systemd

  "Unit name /machine.slice is not valid."

and syslog'ing that error, and then sending back the dbus client a useless
"Input/output error" message :-(

Version-Release number of selected component (if applicable):
systemd-206

(NB, i know this version isn't in RHEL-7 trees yet, but it will be, and so this bug is to just track the problem 206 has)

How reproducible:
Always

Steps to Reproduce:
1.# systemd-nspawn --slice /machine.slice -D /mnt/demo/ -M foo  /bin/sh


Actual results:
  Spawning namespace container on /mnt/demo (console is /dev/pts/5).
  Init process in the container running as PID 32057.
  Failed to register machine: Input/output error
  Container failed with error code 251.


Expected results:
  Spawning namespace container on /mnt/demo (console is /dev/pts/5).
  Init process in the container running as PID 32057.
  Failed to register machine: Unit name /machine.slice is not valid
  Container failed with error code 251.


Additional info:
Reported upstream

http://lists.freedesktop.org/archives/systemd-devel/2013-July/012259.html