qemu.tests: new test to check image backingfile after stream done

new test to verify image file no backingfile after stream done;

changes from v1:
    update test steps in doc string;

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests.cfg: cfg for block_stream.check_backingfile

configuration block for block_stream.check_backingfile test

Signed-off-by: Xu Tian <xutian@redhat.com>