Add a new "iscsi_queue_length()" which will tell us how many commands

are in flight at the moment.
Aside from commands, we also consider the "has not yet connected completely" as being an i/o.

When this command returns 0 it means we are connected ant the iscsi connection is idle, with no commands in flight.

KVM needs a function to detect idleness like this for its block layer io_flush
function.

include/iscsi.h

@@ -47,6 +47,10 @@ int iscsi_which_events(struct iscsi_context *iscsi);
  * file descriptor.
  */
 int iscsi_service(struct iscsi_context *iscsi, int revents);
+/*
+ * How many commands are in flight.
+ */
+int iscsi_queue_length(struct iscsi_context *iscsi);