Description of problem:
When trying to initialize a Lago environment with more then one VM on EL7, Lago fails while running 'virt-sysprep' for some of the VMs while succeeding for others.

Error message example follows:

2015-11-25 13:51:28,256 - root - DEBUG - command stderr: libvirt: XML-RPC error : Failed to connect socket to '/run/user/24044/libvirt/libvirt-sock': No such file or directory
virt-sysprep: error: libguestfs error: could not connect to libvirt (URI = 
qemu:///session): Failed to connect socket to 
'/run/user/24044/libvirt/libvirt-sock': No such file or directory [code=38 
domain=7]

If reporting bugs, run virt-sysprep with debugging enabled and include the 
complete output:

  virt-sysprep -v -x [...]

2015-11-25 13:51:28,256 - root - ERROR - Error while running thread
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/lago/utils.py", line 44, in _ret_via_queue
    queue.put({'return': func()})
  File "/usr/lib/python2.7/site-packages/lago/virt.py", line 123, in <lambda>
    utils.invoke_in_parallel(lambda vm: vm.bootstrap(), self._vms.values())
  File "/usr/lib/python2.7/site-packages/lago/virt.py", line 909, in bootstrap
    ) for i, nic in enumerate(self._spec['nics']) if 'ip' in nic
  File "/usr/lib/python2.7/site-packages/lago/sysprep.py", line 122, in sysprep
    raise RuntimeError('Failed to bootstrap %s' % disk)
RuntimeError: Failed to bootstrap /home/bkorren/src/workspace/jenkins/images/builder_root.qcow2
2015-11-25 13:51:28,266 - root - ERROR - Error occured, aborting
Traceback (most recent call last):
  File "/usr/bin/lagocli", line 532, in main
    func(args)
  File "/usr/bin/lagocli", line 78, in do_init
    prefix.virt_conf(virt_conf, repo, store)
  File "/usr/lib/python2.7/site-packages/lago/__init__.py", line 349, in virt_conf
    env.bootstrap()
  File "/usr/lib/python2.7/site-packages/lago/virt.py", line 123, in bootstrap
    utils.invoke_in_parallel(lambda vm: vm.bootstrap(), self._vms.values())
  File "/usr/lib/python2.7/site-packages/lago/utils.py", line 87, in invoke_in_parallel
    vt.join_all()
  File "/usr/lib/python2.7/site-packages/lago/utils.py", line 44, in _ret_via_queue
    queue.put({'return': func()})
  File "/usr/lib/python2.7/site-packages/lago/virt.py", line 123, in <lambda>
    utils.invoke_in_parallel(lambda vm: vm.bootstrap(), self._vms.values())
  File "/usr/lib/python2.7/site-packages/lago/virt.py", line 909, in bootstrap
    ) for i, nic in enumerate(self._spec['nics']) if 'ip' in nic
  File "/usr/lib/python2.7/site-packages/lago/sysprep.py", line 122, in sysprep
    raise RuntimeError('Failed to bootstrap %s' % disk)
RuntimeError: Failed to bootstrap /home/bkorren/src/workspace/jenkins/images/builder_root.qcow2



Steps to Reproduce:
1. Create a Lago JSON file setting up more then one VM
2. run 'lagocli init' on EL7 with that JSON

Actual results:
Lago crashes with the error above and erases the prefix it began to create.

Expected results:
The Lago environment should be initialised.

Additional info:
Preventing Lago from bootstrapping VMs in parallel worked-around the issue.
To do that the VirtEnv.bootstrap method was patched with the following code:

    def bootstrap(self):
        #utils.invoke_in_parallel(lambda vm: vm.bootstrap(), self._vms.values())
        for vm in self._vms.values():
	    vm.bootstrap()