qemu-img: make progress output more accurate during convert

the progress output is very bumpy if the input images contains
a significant portion of unallocated sectors. This patch
checks how much sectors are allocated a priori if progress
output is selected.

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: introduce bdrv_co_{readv, writev, flush_to_disk}

this converts read, write and flush functions from aio to coroutines
eliminating almost 200 lines of code.

The requirement for libiscsi is bumped to version 1.4.0 which was
released in may 2012.

Signed-off-by: Peter Lieven <pl@kamp.de>
qemu-img: fix for failed bdrv_get_info calls in convert

if bdrv_get_info fails the contents of bdi are undefined.

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