qapi: Add visitor for implicit structs

qapi: Add visitor for implicit structs

These can be used when an embedded struct is parsed and members not
belonging to the struct may be present in the input (e.g. parsing a
flat namespace QMP union, where fields from both the base and one
of the alternative types are mixed in the JSON object)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>