qdev: allow setting drive property for realized device

qdev: allow setting drive property for realized device

We need an ability to insert filters above top block node, attached to
block device. It can't be achieved with blockdev-reopen command. So, we
want do it with help of qom-set.

Intended usage:

Assume there is a node A that is attached to some guest device.

1. blockdev-add to create a filter node B that has A as its child.

2. qom-set to change the node attached to the guest device’s
   BlockBackend from A to B.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-5-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>