Add string_of_location function

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Fix Update.split, to avoid generating -ve offsets

Obviously when splitting an update starting at offset x
into sectors, we should start at (x/sector_size * sector_size)

This fixes the broken unit test.

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