mirror: support arbitrarily-sized iterations

Yet another optimization is to extend the mirroring iteration to include more
adjacent dirty blocks.  This limits the number of I/O operations and makes
mirroring efficient even with a small granularity.  Most of the infrastructure
is already in place; we only need to put a loop around the computation of
the origin and sector count of the iteration.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/trace-events b/trace-events
index b4a23cd..2b28076 100644
--- a/trace-events
+++ b/trace-events
@@ -89,6 +89,7 @@
 mirror_yield(void *s, int64_t cnt, int buf_free_count, int in_flight) "s %p dirty count %"PRId64" free buffers %d in_flight %d"
 mirror_yield_in_flight(void *s, int64_t sector_num, int in_flight) "s %p sector_num %"PRId64" in_flight %d"
 mirror_yield_buf_busy(void *s, int nb_chunks, int in_flight) "s %p requested chunks %d in_flight %d"
+mirror_break_buf_busy(void *s, int nb_chunks, int in_flight) "s %p requested chunks %d in_flight %d"
 
 # blockdev.c
 qmp_block_job_cancel(void *job) "job %p"