For a single poll of the completion queue, if the user
submits I/O from within their completion callback and their
completion callback is particularly slow to execute, the loop
could potentially continue forever. To support this, we
need to limit the number of completions we'll process
in one batch.
Change-Id: If6bae47e52b36347dbe5622ace68c866ee88a0b2
Signed-off-by: Ben Walker <benjamin.walker@intel.com>