virsh: cmdEvent: Rewrite questionable event registration

virsh: cmdEvent: Rewrite questionable event registration

The code registering the event handlers in 'cmdEvent' had too many
blocks of code conditional on whether just one event is being listened
to or all events.

The code can be greatly simplified by uniting the code paths and having
only one branch when filling the list of events we want to listen for.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>