vzct: Container with VNC password in domain.xml cannot start

>Description of problem:
Container with plain-text VNC password specified in domain.xml cannot start due to libvirt internal error:

error: Failed to start domain 100500
error: internal error: cannot mix caller fds with blocking execution

>How reproducible:
100%

>Steps to Reproduce:
1. create container
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vzctl create 100500
===========

2. virsh edit container's domain config
===========
virsh -c 'vzct:///system' edit 100500
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3. make sure to specify vnc password
===========
...
    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' passwd='MyPasswd'>
      <listen type='address' address='127.0.0.1'/>
    </graphics>
  </devices>
</domain>
===========

>Actual results:
===========
[root@chi-solus4 ~]# virsh -c 'vzct:///system&#39; start 100500
error: Failed to start domain 100500
error: internal error: cannot mix caller fds with blocking execution
===========

>Expected results:
container starts successfully

>Additional info (see https://openvz.org/Reporting_OpenVZ_problem):
This bug prevents provisioning, hence blocker priority.

Bug was introduced in commit https://src.openvz.org/projects/OVZ/repos/libvirt/commits/1e34b0dbb6a392dbb4d836865fe205d35591e11a

We fail in https://src.openvz.org/projects/OVZ/repos/libvirt/browse/src/util/vircommand.c?at=refs%2Fheads%2Fvz7.5-u4#2451

Simultaneous use of infd, errfd and outfd may lead to deadlocks according to the comment in virCommandRun, hence libvirt fails such invocations pre-emptively. Prior to the commit, errfd and outfd were not set, thus async_io was equal to false, hence problem didn't exist in the first place.

gdb output from 7.5.4 to illustrate current state in virCommandRun:
===========
(gdb) p cmd->infd
$3 = -1
(gdb) p cmd->outfdptr
$4 = (int *) 0x7f58d5f8b7d0
(gdb) p cmd->outbuf
$5 = (char **) 0x0
(gdb) p cmd->errbuf
$6 = (char **) 0x0
(gdb) p cmd->inbuf
$7 = 0x7f58b06f92d0 "MyPasswd"
===========