qapi-schema: Make block-core.json self-contained

Except for block-core.json, the sub-schemas are self-contained: if
they use a symbol defined in another sub-schema, they include that
sub-schema.  To check, feed the sub-schema to qapi2texi (or any other
QAPI generator) along with the pragma from qapi-schema.json.

Fix up things to make block-core.json self-contained, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503602048-12268-15-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
