qapi: Add support for { 'href' : 'sub_module_path' } as a way to handle JSON submodules

Every top level dictionary containing an href key will be replaced by the
elements of the list resulting from the parsing of the href key value submodule.

This will allow for easy splitting of qapi-schema.json in multiple submodules.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
qapi: Separate the block qapi decriptions in their own submodules.

Two submodules are extracted: the core submodule made to be used with qemu
block utilities (qemu-nbd etc) and the block submodule for inclusion by the main
qapi-schema.json.

Signed-off-by: Benoit Canet <benoit@irqsave.net>