block/iscsi: handle zero events from iscsi_which_events

block/iscsi: handle zero events from iscsi_which_events

newer libiscsi versions may return zero events from iscsi_which_events.

In this case iscsi_service will return immediately without any progress.
To avoid busy waiting for iscsi_which_events to change we deregister all
read and write handlers in this case and schedule a timer to periodically
check iscsi_which_events for changed events.

Next libiscsi version will introduce async reconnects and zero events
are returned while libiscsi is waiting for a reconnect retry.

Signed-off-by: Peter Lieven <[email protected]>
Message-id: 1428437295[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>