qemu: caps: Probe events from 'query-qmp-schema' rather than 'query-events'

qemu: caps: Probe events from 'query-qmp-schema' rather than 'query-events'

QEMU plans to deprecate 'query-events' as it's non-extensible. Events
are also described by 'query-qmp-schema' so we can use that one instead.

This patch adds detection of events to
virQEMUCapsProbeQMPSchemaCapabilities using the same structure declaring
them for the old approach (virQEMUCapsEvents). This is possible as the
name is the same in the QMP schema and our detector supports that
trivially.

For any complex queries virQEMUCapsQMPSchemaQueries can be used in the
future.

For now we still call 'query-events' and discard the result so that it's
obvious that the tests pass. This will be cleaned up later.

https://bugzilla.redhat.com/show_bug.cgi?id=1673320

Signed-off-by: Peter Krempa <pkrempa@redhat.com>