Signed-off-by: NWang Shengfang <wangshengfang@huawei.com>

fix bug due to backup non-existent snapshot.
Signed-off-by: Nlolyu <lolyu@redhat.com>

check if rebase with cache mode `none`, flag `O_DIRECT` is on for open
or openat.
Signed-off-by: Nlolyu <lolyu@redhat.com>

1. add support for luks as base image
2. refactor the code with functions provided in
provider/qemu_img_utils.py
Signed-off-by: Nlolyu <lolyu@redhat.com>

test qemu-img commit with option '-b', which specifies the layer to
commit to.
Signed-off-by: Nlolyu <lolyu@redhat.com>