qmp: add qmp-events.txt back

qmp: add qmp-events.txt back

The conversion of events to the QAPI, resulted in the removal of the
docs/qmp/qmp-events.txt file. This was done to avoid having duplicated
information between qmp-events.txt and qapi-event.json.

However, qmp-events.txt contains examples and we're still not sure
how to proper install QAPI docs in the host. To avoid harming users,
it's better to re-add qmp-events.txt for now and deal with the
duplication later.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>