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>
