mirror: support more than one in-flight AIO operation

mirror: support more than one in-flight AIO operation

With AIO support in place, we can start copying more than one chunk
in parallel.  This patch introduces the required infrastructure for
this: the buffer is split into multiple granularity-sized chunks,
and there is a free list to access them.

Because of copy-on-write, a single operation may already require
multiple chunks to be available on the free list.

In addition, two different iterations on the HBitmap may want to
copy the same cluster.  We avoid this by keeping a bitmap of in-flight
I/O operations, and blocking until the previous iteration completes.
This should be a pretty rare occurrence, though; as long as there is
no overlap the next iteration can start before the previous one finishes.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>