I believe I have the information I need.

I have verified that it is possible to use fence_virt to fence between virtual
guests through virtual machine serial port with fence-virt-0.2.3-18.el6.x86_64.

As a setup template I have used KB article:
https://access.redhat.com/solutions/293183 describing how to do the exactly
same thing only instead of virtual ports multicast is used. I used only one
physical machine.

-----

tardis-02#rpm -q fence-virt
fence-virt-0.2.3-18.el6.x86_64

# virsh dumpxml tardis-02-node01
...
<devices>
    ...
    <serial type='unix'>
      <source mode='bind' path='/var/lib/libvirt/fence-sockets/tardis-02-node01'/>
      <target port='1'/>
      <alias name='serial0'/>
    </serial>


# ls -l /var/lib/libvirt/fence-sockets
total 0
srwxr-xr-x. 1 qemu qemu 0 Sep 11 13:03 tardis-02-node01
srwxr-xr-x. 1 qemu qemu 0 Sep 11 13:56 tardis-02-node02


tardis-02# virsh list
 Id    Name                           State
----------------------------------------------------
 8     tardis-02-node01               running
 13    tardis-02-node02               running

tardis-02# cat /etc/fence_virt.conf 
fence_virtd {
    listener = "serial";
    backend = "libvirt";
    module_path = "/usr/lib64/fence-virt";
}

listeners {
    serial {
            mode = "serial";
            path = "/var/lib/libvirt/fence-sockets";
    }
}

backends {
    libvirt {
            uri = "qemu:///system";
    }
}

tardis-02# ps axf | grep fence_virt
 1785 pts/1    Rl+   17:58  |       \_ fence_virtd

tardis-02# ssh tardis-01-node01 'fence_virt -D /dev/ttyS0 -o list'
tardis-02-node01     bd0687e4-1a3f-0c32-a5de-8785f8e0ad60 on
tardis-02-node02     37736a1b-be13-c282-9679-82f7d9e5ac71 on
tardis-02-node03     1b1f0e2f-e0e7-6193-e4f3-2a570bb9e8c0 off

tardis-01# ssh tardis-01-node01 \
'fence_virt -D /dev/ttyS0 -H tardis-02-node02 -o off'

tardis-02# ssh -q tardis-02-node01 'fence_virt -D /dev/ttyS0 -H tardis-02-node02 -o off'
tardis-02# echo $?
0

tardis-02# ssh -q tardis-02-node01 'fence_virt -D /dev/ttyS0 -o list'
tardis-02-node01     bd0687e4-1a3f-0c32-a5de-8785f8e0ad60 on
tardis-02-node02     37736a1b-be13-c282-9679-82f7d9e5ac71 off
tardis-02-node03     1b1f0e2f-e0e7-6193-e4f3-2a570bb9e8c0 off

tardis-02# virsh list --all
 Id    Name                           State
----------------------------------------------------
 8     tardis-02-node01               running
 -     tardis-02-node02               shut off
 -     tardis-02-node03               shut off

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2014-1589.html