without that one gets a "failed to lookup datastore X" in the log for
every datastore that is in read-only or offline maintenance mode,
even if they aren't scheduled for GC anyway.
Avoid that by first opening the datastore through a Lookup operation,
and only re-open it as Write op once we know that GC needs to get
scheduled for it.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>