Description of problem:
any use of a non-0 function on a slot should be an error unless function 0 of that slot is also used

Version-Release number of selected component (if applicable):
libvirt-daemon-3.2.0-14.el7.x86_64
qemu-kvm-rhev-2.9.0-12.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.start a vm with 2 virtio disks using the xml:
 <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/RHEL-7.4-x86_64-latest.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/b.img'/>
      <target dev='vdb' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x2'/>
    </disk>

in the guest,only vdb uses the 08 slot and the function is non-0,no device uses the 08 slot with function=0

[root@lmen1 ~]# virsh start test
Domain test started


[root@lmen1 ~]# virsh dumpxml test | grep 0x08
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x2'/>

2.check disks in guest, cannot find disk vdb
in the guest,
#lsblk
NAME          MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vda           252:0    0  10G  0 disk 
├─vda1        252:1    0   1G  0 part /boot
└─vda2        252:2    0   9G  0 part 
  ├─rhel-root 253:0    0   8G  0 lvm  /
  └─rhel-swap 253:1    0   1G  0 lvm  [SWAP]


Actual results:
can start the guest,but can not find the disk

Expected results:
output some error info

Additional info: