Description of problem:
Blockcommit hang with unresponsive NFS storage which also blocks other virsh commands.

Version-Release number of selected component (if applicable):
libvirt-1.3.4-1.el7.x86_64
qemu-kvm-rhev-2.6.0-4.el7.x86_64

How reproducible:
100%

Steps to Reproduce:

1.Soft mount a NFS storage 
#mount|grep nfs 
$IP:/mnt/img on /tmp/zp type nfs4 (rw,relatime,vers=4.0,soft,proto=tcp,......)


2. start a guest like following : 
all the images and backing files are on NFS storage.

 <disk type='file' device='disk' snapshot='no'>
      <driver name='qemu' type='qcow2' cache='none' error_policy='stop' io='threads'/>
      <source file='/tmp/zp/snapshot/d/d' startupPolicy='optional'>
        <seclabel model='selinux' labelskip='yes'/>
      </source>
      <backingStore type='file' index='1'>
        <format type='qcow2'/>
        <source file='/tmp/zp/snapshot/d/../c/c'/>
        <backingStore type='file' index='2'>
          <format type='qcow2'/>
          <source file='/tmp/zp/snapshot/d/../c/../b/b'/>
          <backingStore type='file' index='3'>
            <format type='qcow2'/>
            <source file='/tmp/zp/snapshot/d/../c/../b/../a/a'/>
            <backingStore/>
          </backingStore>
        </backingStore>
      </backingStore>
      <target dev='vda' bus='virtio'/>
      <serial>aca38821-c430-48a1-a932-a4814198f24d</serial>
      <boot order='1'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>

3. disconnect NFS storage.
# iptables -A OUTPUT -d $IP -p tcp --dport 2049 -j DROP

4. do blockcommit, it will hang 
# virsh -k0 blockcommit --active --verbose vm2 vda --shallow --pivot --keep-relative 
......

5. in termianl 2, virsh list guest also hang
#virsh list 
...

6. in termianl 3, execute other virsh commands, it looks like all virsh command would hang. 

7. recover NFS storage and commit again, successfully. 
# virsh -k0 blockcommit --active --verbose vm2 vda --shallow --pivot --keep-relative 
Block commit: [100 %]
Successfully pivoted

Actual results:
As step 4, step 5, for unresposive NFS storage, blockcommit will hang, and also it will block other commands.

Expected results:
These commands won't hang.

Additional info: