We are gradually moving away from sector-based interfaces, towards
byte-based.  Make the change for the last few sector-based calls
into the block layer from the vhdx driver.

Ideally, the vhdx driver should switch to doing everything
byte-based, but that's a more invasive change that requires a
bit more auditing.

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