qapi/block-core.json contains block definitions unrelated to emulation.

qapi/block.json is a superset of the previous and contains definitions related
to emulation.

The purpose of these extractions is to be able to hook qapi/block-core.json
generated code on qemu-nbd.
Signed-off-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>