qcow2: Fix preallocation bdrv_pwrite to wrong file

qcow2: Fix preallocation bdrv_pwrite to wrong file

With an external data file, preallocate_co() must write the final byte
to the external data file, not to the qcow2 image file.

This is harmless for preallocation of newly created images (only the
qcow2 file size is increased to the virtual disk size while it should be
much smaller), but with preallocated resize, it could in theory cause
visible corruption if the metadata of the image is larger than the data
(e.g. lots of bitmaps).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>