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: NJiri Denemark <jdenemar@redhat.com>