Fix iSCSI crash on SG_IO with an iovector

Don't assume that SG_IO is always invoked with a simple buffer,
check the iovec_count and if it is >= 1 then we need to pass an array
of iovectors to libiscsi instead of just a plain buffer.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
