Cope with block devices whose sector size isn't 512 bytes

We need to go back and optimise this more: there is now an
unnecessary data copy.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
fat: fix 'create <filename>'

Previously it would fail when <filename> doesn't exist.. which happens
when using the 'create' subcommand.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Release 0.8.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>