block: Drain requests before swapping nodes in bdrv_swap()

bdrv_swap() requires that there are no requests in flight on either of
the two devices. The request coroutine would work on the wrong
BlockDriverState object (with bs->opaque even being interpreted as a
different type potentially) and all sorts of bad things would result
from this.

The currently existing callers mostly ensure that there is no I/O
pending on nodes that are swapped. In detail, this is:

1. Live snapshots. This goes through qmp_transaction(), which calls
   bdrv_drain_all() before doing anything. The command is executed
   synchronously, so no new I/O can be issued concurrently.

2. snapshot=on in bdrv_open(). We're in the middle of opening the image
   (both the original image and its temporary overlay), so there can't
   be any I/O in flight yet.

3. Mirroring. bdrv_drain() is already used on the source device so that
   the mirror doesn't miss anything. However, the main loop runs between
   that and the bdrv_swap() (which is actually a bug, being addressed in
   another series), so there is a small window in which new I/O might be
   issued that would be in flight during bdrv_swap().

It is safer to just drain the request queue of both devices in
bdrv_swap() instead of relying on callers to do the right thing.

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