wait for vm scopes to be gone

wait for vm scopes to be gone

This should help with the rare case where stop mode backups
fail to restart due to the $vmid.scope not being completely
gone when we want to restart. This queries systemd via dbus,
and if the scope is still there, awaits a UnitRemoved signal
for the scope from dbus.

For now with a 5 second timeout... (given that the processes
are already gone and it's really just waiting for systemd to
wake up, this should be plenty...)

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>