Description of problem:
virtio-serial controller can not be generated automatically when tying the serial char device to a particular controller

Version-Release number of selected component (if applicable):
libvirt-1.2.8-6.el7.x86_64
qemu-kvm-1.5.3-77.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
[root@ibm-x3850x5-06 ~]# virsh dumpxml r7 | grep "<address type='virtio-serial'" -B5

    <serial type='pty'>
      <target port='2'/>
      <address type='virtio-serial' controller='1' bus='0' port='1'/>
    </serial>

[root@ibm-x3850x5-06 ~]# virsh dumpxml r7 | grep "controller type='virtio-serial'" -B3
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='virtio-serial' index='0'>

Actual results:
As shown above steps, libvirt only have the default virtio-serial controller.

Expected results:
1. If the action is accepted by libvirt, should added a new virtio-serial controller automatically
2. If not permitted, libvirt need deny the <address> element to <serial> device.

Additional info:
The serial-console controller can be generated automatically on a channel char device.

[root@ibm-x3850x5-06 ~]# virsh dumpxml r7 | grep virtio-serial
    <controller type='virtio-serial' index='0'>
    <controller type='virtio-serial' index='1'>
      <address type='virtio-serial' controller='1' bus='0' port='3'/>
[root@ibm-x3850x5-06 ~]# 
Changed its controller from 1 to 3, dump its xml again.
[root@ibm-x3850x5-06 ~]# virsh edit r7
Domain r7 XML configuration edited.

[root@ibm-x3850x5-06 ~]# virsh dumpxml r7 | grep virtio-serial
    <controller type='virtio-serial' index='0'>
    <controller type='virtio-serial' index='1'>
    <controller type='virtio-serial' index='3'>
      <address type='virtio-serial' controller='3' bus='0' port='3'/>