virsh: Fix *-event error string

virsh: Fix *-event error string

When using
virsh net-event non-existing-net
the error message says that 'either --list or event type is required'
This is misleading as 'virsh net-event $valid-event-type' is not going
to work either. What is expected is 'virsh net-event --event
$valid-event-type'

This commit fixes the string in pool-event, nodedev-event, event, and
net-event.