block/qapi: Clean up how we print to monitor or stdout

bdrv_snapshot_dump(), bdrv_image_info_specific_dump(),
bdrv_image_info_dump() and their helpers take an fprintf()-like
callback and a FILE * to pass to it.

hmp.c passes monitor_printf() cast to fprintf_function and the current
monitor cast to FILE *.

qemu-img.c and qemu-io-cmds.c pass fprintf and stdout.

The type-punning is technically undefined behaviour, but works in
practice.  Clean up: drop the callback, and call qemu_printf()
instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417191805.28198-8-armbru@redhat.com>

qemu-io-cmds.c

@@ -1699,7 +1699,7 @@ static int info_f(BlockBackend *blk, int argc, char **argv)
     }
     if (spec_info) {
         printf("Format specific information:\n");
-        bdrv_image_info_specific_dump(fprintf, stdout, spec_info);
+        bdrv_image_info_specific_dump(spec_info);
         qapi_free_ImageInfoSpecific(spec_info);
     }