Improve block job rate limiting for small bandwidth values

ratelimit_calculate_delay() previously reset the accounting every time
slice, no matter how much data had been processed before. This had (at
least) two consequences:

1. The minimum speed is rather large, e.g. 5 MiB/s for commit and stream.

   Not sure if there are real-world use cases where this would be a
   problem. Mirroring and backup over a slow link (e.g. DSL) would
   come to mind, though.

2. Tests for block job operations (e.g. cancel) were rather racy

   All block jobs currently use a time slice of 100ms. That's a
   reasonable value to get smooth output during regular
   operation. However this also meant that the state of block jobs
   changed every 100ms, no matter how low the configured limit was. On
   busy hosts, qemu often transferred additional chunks until the test
   case had a chance to cancel the job.

Fix the block job rate limit code to delay for more than one time
slice to address the above issues. To make it easier to handle
oversized chunks we switch the semantics from returning a delay
_before_ the current request to a delay _after_ the current
request. If necessary, this delay consists of multiple time slice
units.

Since the mirror job sends multiple chunks in one go even if the rate
limit was exceeded in between, we need to keep track of the start of
the current time slice so we can correctly re-compute the delay for
the updated amount of data.

The minimum bandwidth now is 1 data unit per time slice. The block
jobs are currently passing the amount of data transferred in sectors
and using 100ms time slices, so this translates to 5120
bytes/second. With chunk sizes usually being O(512KiB), tests have
plenty of time (O(100s)) to operate on block jobs. The chance of a
race condition now is fairly remote, except possibly on insanely
loaded systems.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Message-id: 1467127721-9564-2-git-send-email-silbe@linux.vnet.ibm.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
