mgr/volumes: Use operation type during subvolume open

mgr/volumes: Use operation type during subvolume open

Subvolume open currently takes in 2 optional parameters to
denote desired state and type. This enables the open to
allow the operation to suceed based on the (type, state)
tuple.

Instead, pass an operation type to be performed on a subvolume
during open, and decide internal to a subvolume version if the
operation is allowed based on its state and type.

Also modifies the state machine code, to be more amenable to
modifications and improves redability.

Signed-off-by: Shyamsundar Ranganathan <srangana@redhat.com>