blockdev: acquire aio_context for bitmap add/remove

blockdev: acquire aio_context for bitmap add/remove

When bitmaps are persistent, they may incur a disk read or write when bitmaps
are added or removed. For configurations like virtio-dataplane, failing to
acquire this lock will abort QEMU when disk IO occurs.

We used to acquire aio_context as part of the bitmap lookup, so re-introduce
the lock for just the cases that have an IO penalty. Commit 2119882c removed
these locks, and I failed to notice this when we committed fd5ae4cc, so this
has been broken since persistent bitmaps were introduced.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1672010
Reported-By: Aihua Liang <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 20190218233154[email protected]
Signed-off-by: John Snow <[email protected]>