added: enter_systemd_scope

added: enter_systemd_scope

This essentially performas the task of systemd-run while
also waiting for the job to finish.

With the systemd-run version in jessie we run into a race
condition where the executed process can start forking child
processes before the systemd daemon is done setting up the
scope's cgroups, causing the children to NOT be included in
the cgroups. This means the child processes (in our case
qemu) will not adhere to the limits we want to apply to it
via cgroups.

enter_systemd_scope() performs the setup task of systemd-run
and waits for the job to finish, after this we can spawn the
qemu process without systemd-run.