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 4cc841b5, iscsi: partly
 avoid iovec linearization in iscsi_aio_writev, 2012-11-19 - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>