set 'snapshot-delete' lock when deleting snapshot

set 'snapshot-delete' lock when deleting snapshot

unless we are in cleanup mode from a failed snapshot_create,
in which case the config already has a 'snapshot' lock.

this is necessary to prevent concurrent starting of or other
actions on the guest in the windows inbetween holding the
flock, e.g. when removing the volume snapshot (which can
take a bit of time).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>