devices: graphics: Don't use spice default on xen

This matches what we have historically done, prior to the domcaps
change in 30382d57f. Xen will report spice in domcaps but we should
not let that change our historical default

https://listman.redhat.com/archives/virt-tools-list/2022-February/msg00006.html

Signed-off-by: Cole Robinson <crobinso@redhat.com>

virtinst/devices/graphics.py

@@ -155,6 +155,13 @@ class DeviceGraphics(Device):
     def _default_type(self, guest):
         gtype = guest.default_graphics_type
         log.debug("Using default_graphics=%s", gtype)
+
+        if self.conn.is_xen():
+            # Xen domcaps can advertise spice support, but we have historically
+            # not defaulted to it for xen, so force vnc.
+            log.debug("Not defaulting to spice for xen driver. Using vnc.")
+            gtype = "vnc"
+
         if (gtype == "spice" and
             not guest.lookup_domcaps().supports_graphics_spice()):
             log.debug("spice requested but HV doesn't support it. "