migration/block: rename MAX_INFLIGHT_IO to MAX_IO_BUFFERS

this actually limits (as the original commit mesage suggests) the
number of I/O buffers that can be allocated and not the number
of parallel (inflight) I/O requests.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1520507908-16743-4-git-send-email-pl@kamp.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>