qemumonitorjsontest: Work around deprecation of 'vcpu' argument of 'trace-event-get...

qemumonitorjsontest: Work around deprecation of 'vcpu' argument of 'trace-event-get-state'

'trace-event-get-state' was used for testing schema validation as it had
simple arguments. Now 'vcpu' is optional and deprecated. Fix the test so
that it won't break with upcoming qemu-8.1.

Drop the 'all-attrs' case, as it's not not really testing anything
special and for the 'missing mandatory attr' case use an empty object.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>