qmp: Deprecate query-events in favor of query-qmp-schema

query-events doesn't reflect compile-time configuration.  Instead of
fixing that, deprecate the command in favor of query-qmp-schema.

Libvirt prefers query-qmp-schema as of commit 22d7222ec0 "qemu: caps:
Don't call 'query-events' when we probe events from QMP schema".
It'll be in the next release.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190214152251.2073-18-armbru@redhat.com>