qapi-types.py: Implement 'base' for unions

qapi-types.py: Implement 'base' for unions

The new 'base' key in a union definition refers to a struct type, which
is inlined into the union definition and can represent fields common to
all kinds.

For example the following schema definition...

    { 'type': 'BlockOptionsBase', 'data': { 'read-only': 'bool' } }

    { 'union': 'BlockOptions',
      'base': 'BlockOptionsBase',
      'data': {
          'raw': 'BlockOptionsRaw'
          'qcow2': 'BlockOptionsQcow2'
      } }

...would result in this generated C struct:

    struct BlockOptions
    {
        BlockOptionsKind kind;
        union {
            void *data;
            BlockOptionsRaw * raw;
            BlockOptionsQcow2 * qcow2;
        };
        bool read_only;
    };

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