iotests: Test replacing files with x-blockdev-reopen

This patch adds new tests in which we use x-blockdev-reopen to change
bs->file

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20210610120537.196183-10-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>