When creating a RAW file, we don't take advantage
of clone of btrfs.

Add a VIR_STORAGE_VOL_CREATE_REFLINK flag to request
a reflink copy.
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>