tests: Drop 'props' from object-add calls

The 'props' option has been deprecated in 5.0 in favour of a flattened
object-add command. Time to change our test cases to drop the deprecated
option.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>