qapi: Generate command registration stuff into separate files

Having to include qapi-commands.h just for qmp_init_marshal() is
suboptimal.  Generate it into separate files.  This lets
monitor/misc.c, qga/main.c, and the generated qapi-commands-FOO.h
include less.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-4-armbru@redhat.com>
[Typos in docs/devel/qapi-code-gen.txt fixed]
Reviewed-by: Eric Blake <eblake@redhat.com>