block: Add bitmap successors

A bitmap successor is an anonymous BdrvDirtyBitmap that is intended to
be created just prior to a sensitive operation (e.g. Incremental Backup)
that can either succeed or fail, but during the course of which we still
want a bitmap tracking writes.

On creating a successor, we "freeze" the parent bitmap which prevents
its deletion, enabling, anonymization, or creating a bitmap with the
same name.

On success, the parent bitmap can "abdicate" responsibility to the
successor, which will inherit its name. The successor will have been
tracking writes during the course of the backup operation. The parent
will be safely deleted.

On failure, we can "reclaim" the successor from the parent, unifying
them such that the resulting bitmap describes all writes occurring since
the last successful backup, for instance. Reclamation will thaw the
parent, but not explicitly re-enable it.

BdrvDirtyBitmap operations that target a single bitmap are protected
by assertions that the bitmap is not frozen and/or disabled.

BdrvDirtyBitmap operations that target a group of bitmaps, such as
bdrv_{set,reset}_dirty will ignore frozen/disabled drives with a
conditional instead.

Internal functions that enable/disable dirty bitmaps have assertions
added to them to prevent modifying frozen bitmaps.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1429314609-29776-10-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
