mirror: Speed up bitmap initial scanning

Limiting to sectors_per_chunk for each bdrv_is_allocated_above is slow,
because the underlying protocol driver would issue much more queries
than necessary. We should coalesce the query.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: <1436413678-7114-4-git-send-email-famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>