[PATCH 2/4] ui/console: only walk QemuGraphicConsoles in qemu_console_is_multihead()

[Laszlo Ersek <lersek@redhat.com>]

qemu_console_is_multihead() declares the console "c" a "multihead" console
if there are two different consoles in the system that (a) both reference
"c->device", and (b) have different "c->head" numbers. In effect, if at
least two consoles exist that are different heads of the same device that
underlies "c".

Commit 58d5870845c6 ("ui/console: move graphic fields to
QemuGraphicConsole", 2023-09-04) pushed the "device" and "head" members
from the QemuConsole base class down to the QemuGraphicConsole subclass,
adjusting the referring QOM properties accordingly as well. As a result,
the "device" property lookup in qemu_console_is_multihead() now crashes,
in case the candidate console being investigated for criterion (a) is not
a QemuGraphicConsole instance:

> Unexpected error in object_property_find_err() at qom/object.c:1314:
> qemu: Property 'qemu-fixed-text-console.device' not found
> Aborted (core dumped)

This is effectively an unchecked downcast. Make it checked: only consider
such console candidates that are themselves QemuGraphicConsole instances.

Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com> (odd fixer:Graphics)
Cc: Gerd Hoffmann <kraxel@redhat.com> (odd fixer:Graphics)
Fixes: 58d5870845c6
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---
 ui/console.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/ui/console.c b/ui/console.c
index adacc3473140..2ee65207b430 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -2373,6 +2373,9 @@ static bool qemu_console_is_multihead(DeviceState *dev)
     uint32_t h;
 
     QTAILQ_FOREACH(con, &consoles, next) {
+        if (!QEMU_IS_GRAPHIC_CONSOLE(con)) {
+            continue;
+        }
         obj = object_property_get_link(OBJECT(con),
                                        "device", &error_abort);
         if (DEVICE(obj) != dev) {