qapi: fix examples of blockdev-add with qcow2

qapi: fix examples of blockdev-add with qcow2

The examples use "qcow2" driver with the wrong member name for
BlockdevRef alternate type. This patch changes all wrong member names
from "file" to "data-file" which is the correct member name in
BlockdevOptionsQcow2 for the BlockdevRef field.

Problem was noticed when using the example as a test case for Go
bindings.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Message-Id: <20220901085840.22520-9-victortoso@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>