Fix: libstonithd: Change stonith-event XML output.

This brings the output more in line with what's happening in
print_stonith_action in crm_mon. The following changes have been made
to the XML output:

• The main XML node is now fence_event instead of stonith-event. This

keeps compatibility with crm_mon's schema.

• The 'when' attribute is now named 'completed'. This is also for

crm_mon compatibility purposes.

• The XML output now includes a new extended-status attribute. This is

because anything that's not failed or done is "pending", and some users
may wish to have more information than that.