qapi/qom: Add ObjectOptions for filter-*

qapi/qom: Add ObjectOptions for filter-*

This adds a QAPI schema for the properties of the filter-* objects.

Some parts of the interface (in particular NetfilterProperties.position)
are very unusual for QAPI, but for now just describe the existing
interface.

net.json can't be included in qom.json because the storage daemon
doesn't have it. NetFilterDirection is still required in the new object
property definitions in qom.json, so move this enum to common.json.

Signed-off-by: Kevin Wolf <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Acked-by: Peter Krempa <[email protected]>
Reviewed-by: Eric Blake <[email protected]>