ui: Make qemu_default_pixman_format() return 0 on unsupported formats

In order to remove the logic for detecting supported shared
pixmap formats from device models, make qemu_default_pixman_format()
capable for failing by returning 0 which is not a possible format
value rather than asserting.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

ui/qemu-pixman.c

@@ -84,7 +84,7 @@ pixman_format_code_t qemu_default_pixman_format(int bpp, bool native_endian)
         break;
         }
     }
-    g_assert_not_reached();
+    return 0;
 }
 
 int qemu_pixman_get_type(int rshift, int gshift, int bshift)