qemu: the actual change of default video devide type selection algorithm

qemu: the actual change of default video devide type selection algorithm

If a graphics device was added to XML that had no video device, libvirt
automatically added a video device which was always of type 'cirrus' on
x86_64, even if the underlying qemu didn't support cirrus.

This patch refines a bit the decision about the type of the video device.
Based on QEMU capabilities, cirrus is still preferred but only added if
QEMU supports it, otherwise VGA is used if supported by QEMU.  There is now
no fallback as libvirt only aspires to generate a basic working config and
leaves anything more specific up to higher-level management tools.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Pavel Mores <pmores@redhat.com>