Description of problem:
SCSI passthrough does not work. Any VM with passthrough SCSI device will fail to start.

Version-Release number of selected component (if applicable):
RHEV 3.6.4

How reproducible:
100%

Steps to Reproduce:
1. Pass an SCSI device to the VM and try to start.

Actual results:
Thread-4102::ERROR::2016-04-09 02:03:01,722::vm::759::virt.vm::(_startUnderlyingVm) vmId=`a774093d-fb09-4fb7-aff3-989d7c3755c1`::The vm start process failed
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 703, in _startUnderlyingVm
    self._run()
  File "/usr/share/vdsm/virt/vm.py", line 1941, in _run
    self._connection.createXML(domxml, flags),
  File "/usr/lib/python2.7/site-packages/vdsm/libvirtconnection.py", line 124, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/utils.py", line 1313, in wrapper
    return func(inst, *args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3611, in createXML
    if ret is None:raise libvirtError('virDomainCreateXML() failed', conn=self)
libvirtError: XML error: 'bus', 'target', and 'unit' must be specified for scsi hostdev source address


Expected results:
VM starts and the SCSI device is present to the guest.

Additional info:
Thread-4102::INFO::2016-04-09 02:03:01,720::vm::1932::virt.vm::(_run) vmId=`a774093d-fb09-4fb7-aff3-989d7c3755c1`::<?xml version="1.0" encoding="utf-8"?>
<domain type="kvm" xmlns:ovirt="http://ovirt.org/vm/tune/1.0">
        <name>test</name>
        <uuid>a774093d-fb09-4fb7-aff3-989d7c3755c1</uuid>
        <memory>3001344</memory>
        <currentMemory>3001344</currentMemory>
        <maxMemory slots="16">4294967296</maxMemory>
        <vcpu current="1">16</vcpu>
        <devices>
                <channel type="unix">
                        <target name="com.redhat.rhevm.vdsm" type="virtio"/>
                        <source mode="bind" path="/var/lib/libvirt/qemu/channels/a774093d-fb09-4fb7-aff3-989d7c3755c1.com.redhat.rhevm.vdsm"/>
                </channel>
                <channel type="unix">
                        <target name="org.qemu.guest_agent.0" type="virtio"/>
                        <source mode="bind" path="/var/lib/libvirt/qemu/channels/a774093d-fb09-4fb7-aff3-989d7c3755c1.org.qemu.guest_agent.0"/>
                </channel>
                <input bus="ps2" type="mouse"/>
                <hostdev managed="no" mode="subsystem" type="scsi">
                        <source>
                                <address bus="0" host="4" lun="0" target="0"/>
                        </source>
                </hostdev>
                <memballoon model="virtio">
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x07" type="pci"/>
                </memballoon>
                <controller index="0" model="virtio-scsi" type="scsi">
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x04" type="pci"/>
                </controller>
                <controller index="0" ports="16" type="virtio-serial">
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x05" type="pci"/>
                </controller>
                <video>
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x02" type="pci"/>
                        <model heads="1" ram="65536" type="qxl" vgamem="16384" vram="8192"/>
                </video>
                <graphics autoport="yes" listen="0" passwd="*****" passwdValidTo="1970-01-01T00:00:01" port="-1" tlsPort="-1" type="spice"/>
                <interface type="bridge">
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x03" type="pci"/>
                        <mac address="00:1a:4a:16:01:51"/>
                        <model type="virtio"/>
                        <source bridge="ovirtmgmt"/>
                        <filterref filter="vdsm-no-mac-spoofing"/>
                        <link state="up"/>
                        <bandwidth/>
                </interface>
                <disk device="cdrom" snapshot="no" type="file">
                        <address bus="1" controller="0" target="0" type="drive" unit="0"/>
                        <source file="" startupPolicy="optional"/>
                        <target bus="ide" dev="hdc"/>
                        <readonly/>
                        <serial/>
                </disk>
                <disk device="disk" snapshot="no" type="file">
                        <address bus="0x00" domain="0x0000" function="0x0" slot="0x06" type="pci"/>
                        <source file="/rhev/data-center/00000001-0001-0001-0001-00000000022f/9f9069ba-8221-4fad-9ae7-e50a604f0881/images/485dcbe8-bae7-4fe6-be8f-c7e0b57dcc8c/bff788be-6c63-46a7-bbc5-af44b3e3e62b"/>
                        <target bus="virtio" dev="vda"/>
                        <serial>485dcbe8-bae7-4fe6-be8f-c7e0b57dcc8c</serial>
                        <boot order="1"/>
                        <driver cache="none" error_policy="stop" io="threads" name="qemu" type="raw"/>
                </disk>
                <channel type="spicevmc">
                        <target name="com.redhat.spice.0" type="virtio"/>
                </channel>
        </devices>
        <metadata>
                <ovirt:qos/>
        </metadata>
        <os>
                <type arch="x86_64" machine="pc-i440fx-rhel7.2.0">hvm</type>
                <smbios mode="sysinfo"/>
        </os>
        <sysinfo type="smbios">
                <system>
                        <entry name="manufacturer">Red Hat</entry>
                        <entry name="product">RHEV Hypervisor</entry>
                        <entry name="version">7.2-9.el7</entry>
                        <entry name="serial">32393735-3733-5A43-4A32-32343030574E</entry>
                        <entry name="uuid">a774093d-fb09-4fb7-aff3-989d7c3755c1</entry>
                </system>
        </sysinfo>
        <clock adjustment="0" offset="variable">
                <timer name="rtc" tickpolicy="catchup"/>
                <timer name="pit" tickpolicy="delay"/>
                <timer name="hpet" present="no"/>
        </clock>
        <features>
                <acpi/>
        </features>
        <cpu match="exact">
                <model>Westmere</model>
                <topology cores="1" sockets="16" threads="1"/>
                <numa>
                        <cell cpus="0" memory="3001344"/>
                </numa>
        </cpu>
</domain>