prefer storage_check_enabled over storage_check_node

prefer storage_check_enabled over storage_check_node

storage_check_enabled simply checks for the 'disable' option and then calls
storage_check_node.

While not strictly necessary for a second call where only the storage differs,
e.g. in case of clone, it is more future-proof: if support for a target storage
is added at some point, it might be easy to miss adapting the call.

For the migration checks, the situation is improved by now always catching
disabled (target) storages.

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