config: activate affected storages for snapshot operations

config: activate affected storages for snapshot operations

For snapshot creation, the storage for the vmstate file is activated
via vdisk_alloc when the state file is created.

Do not activate the volumes themselves, as that has unnecessary side
effects (e.g. waiting for zvol device link for ZFS, mapping the volume
for RBD). If a storage can only do snapshot operations on a volume
that has been activated, it needs to activate the volume itself.

The actual implementation will be in the plugins to be able to skip
CD ROM drives and bind-mounts, etc.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>