iotests: Refactor blockdev-reopen test for iothreads

iotests: Refactor blockdev-reopen test for iothreads

We'll want to test more than one successful case in the future, so
prepare the test for that by a refactoring that runs each scenario in a
separate VM.

test_iothreads_switch_{backing,overlay} currently produce errors, but
these are cases that should actually work, by switching either the
backing file node or the overlay node to the AioContext of the other
node.

Signed-off-by: Kevin Wolf <[email protected]>
Tested-by: Peter Krempa <[email protected]>
Message-Id: <20200306141413[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>