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,
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, disabled storages are now always caught.

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