qxl: don't assert() if device isn't yet initialized

If the PCI BAR isn't yet mapped or was unmapped, QXL_IO_SET_MODE will
assert(). Instead, report a guest bug and keep going.

This can be reproduced with:

cat << EOF | ./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

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1829

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/display/qxl.c

@@ -1591,7 +1591,10 @@ static void qxl_set_mode(PCIQXLDevice *d, unsigned int modenr, int loadvm)
     }
 
     d->guest_slots[0].slot = slot;
-    assert(qxl_add_memslot(d, 0, devmem, QXL_SYNC) == 0);
+    if (qxl_add_memslot(d, 0, devmem, QXL_SYNC) != 0) {
+        qxl_set_guest_bug(d, "device isn't initialized yet");
+        return;
+    }
 
     d->guest_primary.surface = surface;
     qxl_create_guest_primary(d, 0, QXL_SYNC);