This patch adds two bdrv-drain tests for what happens if some BDS goes
away during the drainage.

The basic idea is that you have a parent BDS with some child nodes.
Then, you drain one of the children.  Because of that, the party who
actually owns the parent decides to (A) delete it, or (B) detach all its
children from it -- both while the child is still being drained.

A real-world case where this can happen is the mirror block job, which
may exit if you drain one of its children.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>