Description:

Trying to start a qemu/kvm guest () it immediately fails with following message:

error: Failed to start domain <guest>
error: internal error process exited while connecting to monitor: chardev: opening backend "pty" failed

Additional info:
* package version(s): 1.0.3-1rc2 and 1.0.3-2rc2 (newest as of writing this). Older versions (=<1.0.2-3) work fine.

Steps to reproduce:

Basically, just start the libvirtd service and do a

virsh start <guest>

for some existing guest; but any fronted (virt-manager, virt-inst) reproduces the issue.

My observations:
the following are irrelevant:
- whether systemd or sysvinit is used
- whether qemu or qemu-kvm is used
- cgroup tunings

The scenario is: qemu (ran as nobody) tries to set up a pseudo-terminal.
It calls openpty(), which internally invokes a setuid helper, pt_chown.
pt_chown is expected to make the ptmx-asssinged new pty owned by the caller.
However, the respective chown(2) call fails with EPERM, despite the 0 effective uid.
It might be something with capabilities... see the /proc/<pid>/status file of such a
pt_chown proccess attached. (Nb. when su-ing to "nobody" in root shell, there is no problem
with pt_chown, it succeeds).