block/iscsi: drop unused IscsiAIOCB->buf field

block/iscsi: drop unused IscsiAIOCB->buf field

The IscsiAIOCB->buf field has not been used since commit
e49ab19fcaa617ad6cdfe1ac401327326b6a2552 ("block/iscsi: bump libiscsi
requirement to 1.9.0").  It used to be a linear buffer for old libiscsi
versions that didn't support scatter-gather.  The minimum libiscsi
version supports scatter-gather so we don't linearize buffers anymore.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20180203061621[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>