We are gradually moving away from sector-based interfaces, towards
byte-based.  The qcow driver is now ready to fully utilize the
byte-based callback interface, as long as we override the default
alignment to still be 512 (needed at least for asserts present
because of encryption, but easier to do everywhere than to audit
which sub-sector requests are handled correctly, especially since
we no longer recommend qcow for new disk images).

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