Description of problem:
Currently virtio console can never generate virtio serial address.

Version-Release number of selected component (if applicable):
libvirt-3.2.0-3.el7.x86_64
qemu-kvm-rhev-2.9.0-2.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. start a guest with following virtio console 
# virsh dumpxml r74 |grep console -A 5
    <console type='pty'>
      <target type='virtio' port='0'/>
    </console>

# virsh start r74
Domain r74 started

# virsh dumpxml r74 |grep console -A 5
    <console type='pty' tty='/dev/pts/4'>
      <source path='/dev/pts/4'/>
      <target type='virtio' port='0'/>
      <alias name='console0'/>
    </console>

2. Hotplug a virtio console  
# cat console.xml 
  <console type='pty'>
      <target type='virtio' port='0'/>
    </console>

# virsh attach-device r74 console.xml 
Device attached successfully

# virsh dumpxml r74 |grep console -A 5
    <console type='pty' tty='/dev/pts/5'>
      <source path='/dev/pts/5'/>
      <target type='virtio' port='0'/>
      <alias name='console1'/>
    </console>

3. Adding virtio-serial address for virtio console via editing vm xml.
#virsh edit r74
...
  <console type='pty'>
      <target type='virtio' port='0'/>
      <address type='virtio-serial' controller='0' bus='0' port='3'/>
    </console>
...

#virsh start r74
Domain r74 started

# virsh dumpxml r74|grep console -A 3
    <console type='pty' tty='/dev/pts/4'>
      <source path='/dev/pts/4'/>
      <target type='virtio' port='0'/>
      <alias name='console0'/>
      <address type='virtio-serial' controller='0' bus='0' port='3'/>
    </console>

Check qemu cmdline :
......
-chardev pty,id=charconsole0 
-device virtconsole,bus=virtio-serial0.0,nr=3,chardev=charconsole0,id=console0 
......

Actual results:
As step 1 and step 2 virtio console cannot generated address.

Expected results:
As step 3, virtio console should have a virtio-serial address.

Additional info:
This is a seperated bug from https://bugzilla.redhat.com/show_bug.cgi?id=1387666#c6