virsh: Move 'cmdEvent' and all of its machinery to virsh-domain-event.c

virsh: Move 'cmdEvent' and all of its machinery to virsh-domain-event.c

'cmdEvent' along with all the helper functions it needs is ~950 LOC.
Move it out from virsh-domain.c to virsh-domain-event.c along with the
completer function so that the new module doesn't have to expose any new
types.

Semantically this creates a new category in 'virsh help' but all other
behaviour stays the same.

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