iscsi: do not leak acb->buf when commands are aborted

acb->buf is freed in the WRITE(16) callback, but this may not
get called at all when commands are aborted.  Add another
free in the ABORT TASK callback, which requires setting acb->buf
to NULL everywhere.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iscsi: add support for iovectors

This patch adds support for directly passing the iovec
array from QEMUIOVector if libiscsi supports it (1.8.0
or newer).

Signed-off-by: Peter Lieven <pl@kamp.de>
[Preserve the improvements from commit 4cc841b, iscsi: partly
 avoid iovec linearization in iscsi_aio_writev, 2012-11-19 - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>