vga: don't abort when adding a duplicate isa-vga device

If users try to add an isa-vga device that was already registered,
still in command line, qemu will crash:

$ qemu-system-mips64el -M pica61 -device isa-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)

That particular board registers the device automaticaly, so it's
not obvious that a VGA device already exists. This patch changes
this behavior by displaying a message and exiting without crashing.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/44
Signed-off-by: Jose R. Ziviani <jziviani@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210817192629.12755-1-jziviani@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/display/vga-isa.c

@@ -33,6 +33,7 @@
 #include "hw/loader.h"
 #include "hw/qdev-properties.h"
 #include "qom/object.h"
+#include "qapi/error.h"
 
 #define TYPE_ISA_VGA "isa-vga"
 OBJECT_DECLARE_SIMPLE_TYPE(ISAVGAState, ISA_VGA)
@@ -61,6 +62,15 @@ static void vga_isa_realizefn(DeviceState *dev, Error **errp)
     MemoryRegion *vga_io_memory;
     const MemoryRegionPortio *vga_ports, *vbe_ports;
 
+    /*
+     * make sure this device is not being added twice, if so
+     * exit without crashing qemu
+     */
+    if (object_resolve_path_type("", TYPE_ISA_VGA, NULL)) {
+        error_setg(errp, "at most one %s device is permitted", TYPE_ISA_VGA);
+        return;
+    }
+
     s->global_vmstate = true;
     vga_common_init(s, OBJECT(dev));
     s->legacy_address_space = isa_address_space(isadev);