Comment 8 for bug 1496357

Description of problem:
virt-manager should remove "vgamem" from guest automatically when change video from "qxl" to "vga"

Version-Release number of selected component (if applicable):
libvirt-1.2.8-9.el7.x86_64
virt-manager-1.1.0-8.el7.noarch
qemu-kvm-rhev-2.1.2-13.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Install a guest via virt-manager with qxl video:
...
    <video>
      <model type='qxl' ram='65536' vram='65536' vgamem='8192' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
...

2. Launch virt-manager, change guest video from qxl to vga: Double click guest name-->Show virtual hardware details-->change video from qxl to vga-->Apply

Actual results:
After click "Apply" button, get such error message:

Error changing VM configuration: XML error: vgamem attribute only supported for type of qxl

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/addhardware.py", line 1384, in change_config_helper
    vm.redefine_cached()
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 222, in redefine_cached
    self._redefine_xml(xml)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 247, in _redefine_xml
    return self._redefine_helper(origxml, newxml)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 239, in _redefine_helper
    self._define(newxml)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1041, in _define
    self.conn.define_domain(newxml)
  File "/usr/share/virt-manager/virtManager/connection.py", line 694, in define_domain
    return self._backend.defineXML(xml)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3442, in defineXML
    if ret is None:raise libvirtError('virDomainDefineXML() failed', conn=self)
libvirtError: XML error: vgamem attribute only supported for type of qxl

Expected results:
No such error showing and can change guest video successfully.

Additional info:
1. Can also reproduce this issue when change video from qxl to cirrus.