qemu: Use correct flags for ABI stability check in SaveImageUpdateDef

qemu: Use correct flags for ABI stability check in SaveImageUpdateDef

https://bugzilla.redhat.com/show_bug.cgi?id=1183869

Soo. you've successfully started yourself a domain. And since you want
to use it on your host exclusively you are confident enough to
passthrough the host CPU model, like this:

  <cpu mode='host-passthrough'/>

Then, after a while, you want to save the domain into a file (e.g.
virsh save dom dom.save). And here comes the trouble. The file consist
of two parts: Libvirt header (containing domain XML among other
things), and qemu migration data. Now, the domain XML in the header is
formatted using special flags (VIR_DOMAIN_XML_SECURE |
VIR_DOMAIN_XML_UPDATE_CPU | VIR_DOMAIN_XML_INACTIVE |
VIR_DOMAIN_XML_MIGRATABLE).

Then, on your way back from the bar, you think of changing something
in the XML in the saved file (we have a command for it after all), say
listen address for graphics console. So you successfully type in the
command:

  virsh save-image-edit dom.save

Change all the bits, and exit the editor. But instead of success
you're left with sad error message:

  error: unsupported configuration: Target CPU model <null> does not
  match source Pentium Pro

Sigh. Digging into the code you see lines, where we check for ABI
stability. The new XML you've produced is compared with the old one
from the saved file to see if qemu ABI will break or not. Wait, what?
We are using different flags to parse the XML you've provided so we
were just lucky it worked in some cases? Yep, that's right.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>