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>
diff --git a/tests/test-qmp-cmds.c b/tests/test-qmp-cmds.c
index 27b0afe..79507d9 100644
--- a/tests/test-qmp-cmds.c
+++ b/tests/test-qmp-cmds.c
@@ -7,6 +7,7 @@
 #include "tests/test-qapi-types.h"
 #include "tests/test-qapi-visit.h"
 #include "test-qapi-commands.h"
+#include "test-qapi-init-commands.h"
 
 static QmpCommandList qmp_commands;