Revert "qapi: fix examples of blockdev-add with qcow2"

Revert "qapi: fix examples of blockdev-add with qcow2"

This reverts commit b6522938327141235b97ab38e40c6c4512587373.

Kevin Wolf NAKed this patch, because:

    'file' is a required member (defined in BlockdevOptionsGenericFormat),
    removing it makes the example invalid. 'data-file' is only an additional
    optional member to be used for external data files (i.e. when the guest
    data is kept separate from the metadata in the .qcow2 file).

However, it had already been merged then.  Revert.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220930171908.846769-1-armbru@redhat.com>
Reviewed-by: Victor Toso <victortoso@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>