qapi/qom: Add ObjectOptions for iothread

Add an ObjectOptions union that will eventually describe the options of
all user creatable object types. As unions can't exist without any
branches, also add the first object type.

This adds a QAPI schema for the properties of the iothread object.

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>