qcow2: Implement .bdrv_co_preadv()

qcow2: Implement .bdrv_co_preadv()

Reading from qcow2 images is now byte granularity.

Most of the affected code in qcow2 actually gets simpler with this
change. The only exception is encryption, which is fixed on 512 bytes
blocks; in order to keep this working, bs->request_alignment is set for
encrypted images.

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