qapi: create two block related json modules

qapi: create two block related json modules

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: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>