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>