qxl: remove assert in qxl_pre_save.

Since commit 551dbd0 ("migration: check pre_save return in
vmstate_save_state") the pre_save hook can fail.  So lets finally
use that to drop the guest-triggerable assert in qxl_pre_save().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210721093347.338536-2-kraxel@redhat.com>

hw/display/qxl.c

@@ -2283,7 +2283,9 @@ static int qxl_pre_save(void *opaque)
     } else {
         d->last_release_offset = (uint8_t *)d->last_release - ram_start;
     }
-    assert(d->last_release_offset < d->vga.vram_size);
+    if (d->last_release_offset < d->vga.vram_size) {
+        return 1;
+    }
 
     return 0;
 }