vm start: always reset any failed-state of the VM systemd scope

vm start: always reset any failed-state of the VM systemd scope

The scope can get into failed state for some issues like OOM kills of
the whole scope, in that case a user cannot re-start the VM until
they manually reset it.

Do this for now inline to avoid a pve-common bump as done in [0]
(location was suggested by me thinking we could maybe do it over
dbus, but as we have a stop command here already it probably doesn't
matters)

[0]: https://lists.proxmox.com/pipermail/pve-devel/2023-June/057770.html

Originally-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>