Fix migrate set speed doc arg

We used to ignore any fractional part in 0.13, but due to recent
changes (started with 9f9b17a4f0865286391e4d3a0a735230122a2289)
migrate_set_speed will reject the fractional part.

We don't expect existing clients to be relying on this, but we
need to update the documentation to reflect the change.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 3486223..164ecbc 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -510,7 +510,7 @@
 
 Arguments:
 
-- "value": maximum speed, in bytes per second (json-number)
+- "value": maximum speed, in bytes per second (json-int)
 
 Example: