linux-stubdom: use "vga" aka "stdvga" video card

In contrast to the cirrus card emulation the vga card supports more
video modes. Since the qemu gui-agent bug [1] is now fixed we can switch
back to use it.

[1]: QubesOS/qubes-linux-utils@f14637a615

templates/libvirt/xen.xml

@@ -155,11 +155,9 @@
                     {% endif %}
                     />
                 <input type="tablet" bus="usb"/>
-                {% if vm.features.check_with_template('linux-stubdom', True) %}
-                    <video type="cirrus"/>
-                {% else %}
-                    <video type="vga"/>
-                {% endif %}
+                <video>
+                    <model type="vga"/>
+                </video>
                 {% if vm.features.check_with_template('linux-stubdom', True) %}
                     {# TODO only add qubes gui if gui-agent is not installed in HVM #}
                     <graphics type="qubes"/>