Description of problem:
Libvirt should report failure when detach hostdev unsuccessfully.
 
Version-Release number of selected component (if applicable):
libvirt-1.1.1-2.el7.x86_64
qemu-kvm-1.5.2-1.el7.x86_64
kernel-3.10.0-3.el7.x86_64

How reproducible:
100%

Steps:
1. Define a guest with the following hostdev device
<hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
    </hostdev>

2. Execute 2.1 and 2.2 at the same time.
2.1 Start the guest.
[root@SRIOV2 ~]# virsh start r7_g
Domain r7_g started
[root@SRIOV2 ~]# virsh list --all
 Id    Name                           State
----------------------------------------------------
 7     r7_g                           running

2.2 In another terminal，execute below command immediately:
[root@SRIOV2 ~]# virsh detach-device r7_g hostdev.xml
Device detached successfully                           <======= this is issue.

3. Dumpxml domain xml.
[root@SRIOV2 ~]# virsh dumpxml r7_g | grep -A10 hostdev
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </hostdev>

[root@SRIOV2 ~]# virsh dumpxml r7_g --inactive| grep -A10 hostdev
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </hostdev>

4. In guest, to check the nic status, it's in the domain.
[root@test005 ~]# lspci| grep Eth
00:03.0 Ethernet controller: Intel Corporation 82574L Gigabit Network Connection
[root@test005 ~]# ifconfig eth1
eth1      Link encap:Ethernet  HWaddr B4:B5:2F:AF:A1:00  
          inet addr:10.66.71.177  Bcast:10.66.71.255  Mask:255.255.254.0
          inet6 addr: fe80::b6b5:2fff:feaf:a100/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:16 errors:0 dropped:0 overruns:0 frame:0
          TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1932 (1.8 KiB)  TX bytes:1588 (1.5 KiB)
          Interrupt:11 Memory:fc000000-fc020000

[root@test005 ~]# ping 10.66.7.130
PING 10.66.7.130 (10.66.7.130) 56(84) bytes of data.
64 bytes from 10.66.7.130: icmp_seq=1 ttl=62 time=0.324 ms
64 bytes from 10.66.7.130: icmp_seq=2 ttl=62 time=0.282 ms
64 bytes from 10.66.7.130: icmp_seq=3 ttl=62 time=0.402 ms

--- 10.66.7.130 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2391ms
rtt min/avg/max/mdev = 0.282/0.336/0.402/0.049 ms
[root@test005 ~]# 

Actual results:
As step 2.2 shown.

Expected results:
Libvirt should report an error if detach device fail.