iscsi: read unmap info from block limits vpd page

Signed-off-by: Peter Lieven <pl@kamp.de>
Revert "[VC] qemu-img: add verbose sector output"

This reverts commit b748717587f2062fd688240cdeb6449e48b3cd34.
iscsi: let bdrv_create conditionally zero out the device

if the device supports unmapping and unmapped blocks read as
zero ensure that the whole device is unmapped and report
.has_zero_init = 1 in this case to speed up qemu-img convert.

Signed-off-by: Peter Lieven <pl@kamp.de>