throttle: Correct access to wrong BlockBackendPublic structures

throttle: Correct access to wrong BlockBackendPublic structures

In 27ccdd52598290f0f8b58be56e235aff7aebfaf3 the throttling fields were
moved from BlockDriverState to BlockBackend. However in a few cases
the code started using throttling fields from the active BlockBackend
instead of the round-robin token, making the algorithm behave
incorrectly.

This can cause starvation if there's a throttling group with several
drives but only one of them has I/O.

Cc: [email protected]
Reported-by: Paolo Bonzini <[email protected]>
Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>