qemu.tests: update get_backingfile function in block_copy module

raise TestFail doesn't make sense in get_backingfile function, it should
be verify in test script according case;

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