qcow2: Fix src_offset in copy offloading

Not updating src_offset will result in wrong data being written to dst
image.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
3 files changed