[SCSI] virtio-scsi: fix LUNs greater than 255

virtio-scsi needs to report LUNs greater than 256 using the "flat"
format.  Because the Linux SCSI layer just maps the SCSI LUN to
an u32, without any parsing, these end up in the range from 16640
to 32767.  Fix max_lun to account for the possibility that logical
unit numbers are encoded with the "flat" format.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>