iotests: Test handling of AioContexts with some blockdev actions
Includes the following tests:
- Adding a dirty bitmap.
* RHBZ:
1782175
- Starting a drive-mirror to an NBD-backed target.
* RHBZ:
1746217,
1773517
- Aborting an external snapshot transaction.
* RHBZ:
1779036
- Aborting a blockdev backup transaction.
* RHBZ:
1782111
For each one of them, a VM with a number of disks running in an
IOThread AioContext is used.
Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>