migration: Disambiguate MAX_THROTTLE

Migration has a define for MAX_THROTTLE. Update comment to clarify that this is
used for throttling transfer speed. Hopefully this will prevent it from being
confused with a guest cpu throttling entity.
Signed-off-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>

migration/migration.c

@@ -31,7 +31,7 @@
 #include "qapi-event.h"
 #include "qom/cpu.h"
 
-#define MAX_THROTTLE  (32 << 20)      /* Migration speed throttling */
+#define MAX_THROTTLE  (32 << 20)      /* Migration transfer speed throttling */
 
 /* Amount of time to allocate to each "chunk" of bandwidth-throttled
  * data. */