Description of problem:
Trying to run VM with emulated scsi tape device (https://bugzilla.redhat.com/show_bug.cgi?id=1103739#c10) attached, failed with the next event:
"VM rhel7_amd_cuda is down with error. Exit message: XML error: 'bus', 'target', and 'unit' must be specified for scsi hostdev source address."

Version-Release number of selected component (if applicable):
rhevm-3.6.0-0.18.el6
vdsm-4.17.8-1.el7ev.noarch
qemu-kvm-rhev-2.3.0-24.el7.x86_64
sanlock-3.2.4-1.el7.x86_64
libvirt-client-1.2.17-5.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Create emulated scsi tape device on host (from bug https://bugzilla.redhat.com/show_bug.cgi?id=1103739#c10 comment):
- Install scsi-target-utils, iscsi-initiator-utils and lsscsi on host.
- Prepare the emulate scsi tape device. 
# modprobe sg
# service tgtd start
# tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.st:tape:sttarget1 
# tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL 
# dd if=/dev/zero of=/root/tape.device bs=1M count=100 
t# gtadm --lld iscsi --op new --mode logicalunit --tid 1 --device-type tape --lun 1 -b /root/tape.device
# tgtadm --lld iscsi --op show --mode target
# iscsiadm -m discovery -t st -p 127.0.0.1 127.0.0.1:3260,1 
# iqn.st:tape:sttarget1
# iscsiadm -m node -l
- Verify login successful.
- Verify virtual tape attached:
# lsscsi | grep -w tape
# cat /proc/scsi/scsi 

2. From webadmin Refresh host capabilities and attach host emulated device to VM.
3. Try to run VM.

Actual results:
VM failed to run and the next event logged: 
VM rhel7_amd_cuda is down with error. Exit message: XML error: 'bus', 'target', and 'unit' must be specified for scsi hostdev source address.

Expected results:
VM should run with wmulated device attached to it.

Additional info:
engine.log and vdsm.log attached.