storage: Fix iscsi-direct volume size for volumes > 4GiB
    
    Both block_size and nb_block are unit32_t and multiplying them overflows
    at 4GiB.
    
    Moreover, the iscsi_*10_* APIs use 32bit number of blocks and thus they
    can only address images up to 2TiB with 512B blocks. Let's use 64b
    iscsi_*16_* APIs instead.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>