fixup patch "ide: avoid potential deadlock when draining during trim"

The patch was incomplete and (re-)introduced an issue with a potential
failing assertion upon cancelation of the DMA request.

There is a patch on qemu-devel now[0], and it's the same as this one
code-wise (except for comments). But the discussion is still ongoing.
While there shouldn't be a real issue with the patch, there might be
better approaches. The plan is to use this as a stop-gap for now and
pick up the proper solution once it's ready.

[0]: https://lists.nongnu.org/archive/html/qemu-devel/2023-03/msg03325.html

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>