iscsi: Use bs->sg for everything else than disks

The current iscsi block driver code makes the rather arbitrary decision
that TYPE_MEDIUM_CHANGER and TYPE_TAPE devices have bs->sg = 1 and all
other device types are disks.

Instead of this, check for TYPE_DISK to expose the disk interface and
make everything else bs->sg = 1. In particular, this includes devices
with TYPE_STORAGE_ARRAY, which is what LUN 0 of an iscsi target is.
(See https://bugzilla.redhat.com/show_bug.cgi?id=1067784 for the exact
scenario.)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

block/iscsi.c

diff --git a/block/iscsi.c b/block/iscsi.c
index 0a15f53..b490e98 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1231,12 +1231,11 @@
     bs->total_sectors = sector_lun2qemu(iscsilun->num_blocks, iscsilun);
     bs->request_alignment = iscsilun->block_size;
 
-    /* Medium changer or tape. We dont have any emulation for this so this must
-     * be sg ioctl compatible. We force it to be sg, otherwise qemu will try
-     * to read from the device to guess the image format.
+    /* We don't have any emulation for devices other than disks and CD-ROMs, so
+     * this must be sg ioctl compatible. We force it to be sg, otherwise qemu
+     * will try to read from the device to guess the image format.
      */
-    if (iscsilun->type == TYPE_MEDIUM_CHANGER ||
-        iscsilun->type == TYPE_TAPE) {
+    if (iscsilun->type != TYPE_DISK && iscsilun->type != TYPE_ROM) {
         bs->sg = 1;
     }