fix #3030: always activate volumes in storage_migrate

fix #3030: always activate volumes in storage_migrate

AFAICT the snapshot activation is not necessary for our plugins at the moment,
but it doesn't really hurt and might be relevant in the future or for external
plugins.

Deactivating volumes is up to the caller, because for example, for replication
on a running guest, we obviously don't want to deactivate volumes.

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