Description of problem:
Attach interface with type of hostdev supports options of --model and --target

Version-Release number of selected component (if applicable):
libvirt-1.3.1-1.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Setup a SR-IOV, then start a guest.
2.Attach an interface with type of hostdev and --target option, it will succeed.

[root@ibm-x3850x5-05 ~]# virsh attach-interface rhel7.2 hostdev 0000:86:10.0 --target kshdfkjdf --managed --persistent
Interface attached successfully

[root@ibm-x3850x5-05 ~]# virsh dumpxml rhel7.2 | grep /interface -B8
.....
   <interface type='hostdev' managed='yes'>
      <mac address='52:54:00:2e:cd:0b'/>
      <driver name='vfio'/>
      <source>
        <address type='pci' domain='0x0000' bus='0x86' slot='0x10' function='0x0'/>
      </source>
      <target dev='kshdfkjdf'/>
      <alias name='hostdev0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
    </interface>

3.Attach an interface with type of hostdev and --model option, it will succeed.

[root@ibm-x3850x5-05 ~]# virsh attach-interface rhel7.2 hostdev 0000:86:10.1 --model e1000 --managed --persistent
Interface attached successfully

[root@ibm-x3850x5-05 ~]# virsh dumpxml rhel7.2 | grep /interface -B9
.......
    <interface type='hostdev' managed='yes'>
      <mac address='52:54:00:66:b1:ea'/>
      <driver name='vfio'/>
      <source>
        <address type='pci' domain='0x0000' bus='0x86' slot='0x10' function='0x1'/>
      </source>
      <model type='e1000'/>
      <alias name='hostdev1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
    </interface>

4. Validate the xml with all the interface above, it will succeed.
[root@ibm-x3850x5-05 qemu]# pwd
/etc/libvirt/qemu
[root@ibm-x3850x5-05 qemu]# virt-xml-validate rhel7.2.xml
rhel7.2.xml validates
[root@ibm-x3850x5-05 qemu]# echo $?
0
5. On the guest,check the network device
guest# lspci | grep -i eth
00:03.0 Ethernet controller: Red Hat, Inc Virtio network device
00:0c.0 Ethernet controller: Intel Corporation 82599 Ethernet Controller Virtual Function (rev 01)
00:0d.0 Ethernet controller: Intel Corporation 82599 Ethernet Controller Virtual Function (rev 01)
guest# 

Actual results:
The option "--model" or "--target" is invalid for hostdev type interface, but they can be specified with no reminder or error. 

Expected results:
The interface of hostdev should not support a "model type" or "target dev" specified as they are invalid. Or there should be a statement about this.


Addtional info:
N/A