Xen: Retry plugin call after connection reset

When using the call_plugin_serialized_with_retry session method it will
retry if the plugin raises a RetryableError, but not if it loses the
connection to xenapi.  This adds a retry for the 'Connection reset by
peer' error that is occasionally seen.

Change-Id: Ibb36a0170e0e807ad7a4198df7b6e574c4279bed
Closes-Bug: #1200591
Merge "Xen: Retry plugin call after connection reset"