qemu-io: Switch 'alloc' command to byte-based length

For the 'alloc' command, accepting an offset in bytes but a length
in sectors, and reporting output in sectors, is confusing.  Do
everything in bytes, and adjust the expected output accordingly.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170429191419.30051-3-eblake@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>