Add support to activate LVM snapshot

[skyxx]

Request description

Hi, it will be nice to add support activate + mount existing LVM snapshots for storage volumes and containers.

When using LVM storage driver, created snapshots are not accesible as LV is not accesible and not mounted.
Only path /var/lib/incus/storage-pools/<pool_name>/<type>-snapshots/default_<volume_name>/<snapshot_name> is created but no data accessible.

When using BTRFS storage driver after creating snapshot data from it are available automaticaly.
/var/lib/incus/storage-pools/<pool_name>/<type>-snapshots/default_<volume_name>/<snapshot_name>

So something like this will be nice:
incus snapshot activate <instance> <snapshot_name>
incus snapshot deactivate <instance> <snapshot_name>

incus storage volume snapshot activate <pool_name> <volume_name> <snapshot_name>
incus storage volume snapshot deactivate <pool_name> <volume_name> <snapshot_name>

[stgraber]

We're definitely not going to be doing that. In general we never want anyone to directly access files through /var/lib/incus and we definitely will not be adding a user facing API to trigger more mounts happening in there.

For a proper way to access snapshot data, we have: #1174