KVM smoke test hang on Trusty KVM node

Sometimes the KVM smoke test will hang on a KVM node, it looks like the KVM was up and running, but the wait command never returns:

 + uvt-kvm wait bjf-test --insecure --ssh-private-key-file /home/ubuntu/.ssh/id_rsa
 Warning: Permanently added '192.168.122.164' (ECDSA) to the list of known hosts.
 Timer expired (3600 sec.), nuking pid 5029
   ERROR ubuntu_kvm_smoke_test.kvm_smoke_test ubuntu_kvm_smoke_test.kvm_smoke_test timestamp=1544605646 localtime=Dec 12 09:07:26 Test timeout expired, rc=15
  END ERROR ubuntu_kvm_smoke_test.kvm_smoke_test ubuntu_kvm_smoke_test.kvm_smoke_test timestamp=1544605646 localtime=Dec 12 09:07:26

ubuntu@harpo:~$ pstree -p
init(1)-+-acpid(1033)
        |-atd(1025)
        |-autotest-local(1933)
        |-autotest-local(1934)
        |-avahi-daemon(4289)---avahi-daemon(4291)
        |-cron(1024)
        |-dbus-daemon(711)
        |-dnsmasq(4918)
        |-getty(967)
        |-getty(970)
        |-getty(976)
        |-getty(977)
        |-getty(979)
        |-getty(1108)
        |-getty(1115)
        |-irqbalance(1032)
        |-iscsid(910)
        |-iscsid(911)
        |-libvirtd(4768)-+-{libvirtd}(4769)
        | |-{libvirtd}(4770)
        | |-{libvirtd}(4771)
        | |-{libvirtd}(4772)
        | |-{libvirtd}(4773)
        | |-{libvirtd}(4774)
        | |-{libvirtd}(4775)
        | |-{libvirtd}(4776)
        | |-{libvirtd}(4777)
        | `-{libvirtd}(4778)
        |-qemu-system-x86(5127)-+-{qemu-system-x86}(5130)
        | `-{qemu-system-x86}(5132)
        |-rsyslogd(780)-+-{rsyslogd}(783)
        | |-{rsyslogd}(784)
        | `-{rsyslogd}(785)
        |-sshd(1020)-+-sshd(1803)---sshd(1875)---python3(1876)-+-sh(1929)
        | | `-{python3}(1930)
        | `-sshd(5228)---sshd(5327)---bash(5328)---pstree(5344)
        |-sudo(5033)---the-test(5034)---uvt-kvm(5133)---ssh(5192)
        |-systemd-logind(786)
        |-systemd-udevd(452)
        |-upstart-file-br(811)
        |-upstart-socket-(661)
        `-upstart-udev-br(448)

Killing the ssh process invoked by uvt-kvm can unblock it.