virt: when detecting containers and /run/systemd/container cannot be read, check...

virt: when detecting containers and /run/systemd/container cannot be read, check /proc/1/environ

This way, we should be in a slightly better situation if a container is
booted up with only a shell as PID 1. In that case
/run/systemd/container will not be populated, and a check for it hence
be ineffective.

Checking /proc/1/environ doesn't fully fix the problem though, as the
file is only accessible with privileges. This means if PID 1 is not
systemd, and if privileges have been dropped the container detection
will continue to fail.