fix #2043: vm start: always stop existing systemd scopes

fix #2043: vm start: always stop existing systemd scopes

commit 3c23aa808ccc946bad92d9bc63b6f833c61d0f52 tried to fix a issue
where after a stop mode backup a scope could still linger around, but
it actually removed the wrong check. If we want to remove a
lingering, not yet cleaned up, scope we need to check if said scope
exists not if a VM process is still running. While they are corelated
the scope will always get cleaned up _after_ it's processes are gone.

Should fix #2043, but as this is seemingly not that easy to fix one
for all I'll put the should as disclaimer here.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Cc: Wolfgang Bumiller <w.bumiller@proxmox.com>