tests: Test mid-drain bdrv_replace_child_noperm()

tests: Test mid-drain bdrv_replace_child_noperm()

Add a test for what happens when you call bdrv_replace_child_noperm()
for various drain situations ({old,new} child {drained,not drained}).

Most importantly, if both the old and the new child are drained, the
parent must not be undrained at any point.

Signed-off-by: Max Reitz <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>