qxl: create slots on post_load in vga state

RHBZ 740547

If we migrate when the device is in vga state the guest
still believes the slots are created, and will cause operations
that reference the slots, causing a "panic: virtual address out of range"
on the first of them. Easy to see by migrating in vga mode with
a driver loaded, for instance windows cmd window in full screen mode,
and then exiting vga mode back to native mode will cause said panic.

Fixed by doing the slot recreation in post_load for vga mode as well.
Note that compat does not require any changes because it creates it's
only slot by a side effect of QXL_IO_SET_MODE.
Signed-off-by: NAlon Levy <alevy@redhat.com>

hw/qxl.c

@@ -1663,12 +1663,25 @@ static int qxl_pre_load(void *opaque)
     return 0;
 }
 
+static void qxl_create_memslots(PCIQXLDevice *d)
+{
+    int i;
+
+    for (i = 0; i < NUM_MEMSLOTS; i++) {
+        if (!d->guest_slots[i].active) {
+            continue;
+        }
+        dprint(d, 1, "%s: restoring guest slot %d\n", __func__, i);
+        qxl_add_memslot(d, i, 0, QXL_SYNC);
+    }
+}
+
 static int qxl_post_load(void *opaque, int version)
 {
     PCIQXLDevice* d = opaque;
     uint8_t *ram_start = d->vga.vram_ptr;
     QXLCommandExt *cmds;
-    int in, out, i, newmode;
+    int in, out, newmode;
 
     dprint(d, 1, "%s: start\n", __FUNCTION__);
 
@@ -1685,19 +1698,16 @@ static int qxl_post_load(void *opaque, int version)
         qxl_mode_to_string(d->mode));
     newmode = d->mode;
     d->mode = QXL_MODE_UNDEFINED;
+
     switch (newmode) {
     case QXL_MODE_UNDEFINED:
         break;
     case QXL_MODE_VGA:
+        qxl_create_memslots(d);
         qxl_enter_vga_mode(d);
         break;
     case QXL_MODE_NATIVE:
-        for (i = 0; i < NUM_MEMSLOTS; i++) {
-            if (!d->guest_slots[i].active) {
-                continue;
-            }
-            qxl_add_memslot(d, i, 0, QXL_SYNC);
-        }
+        qxl_create_memslots(d);
         qxl_create_guest_primary(d, 1, QXL_SYNC);
 
         /* replay surface-create and cursor-set commands */
@@ -1722,6 +1732,8 @@ static int qxl_post_load(void *opaque, int version)
 
         break;
     case QXL_MODE_COMPAT:
+        /* note: no need to call qxl_create_memslots, qxl_set_mode
+         * creates the mem slot. */
         qxl_set_mode(d, d->shadow_rom.mode, 1);
         break;
     }