Otherwise there is no way to differentiate between global and slot 
specific logging, so for example 

vga dirty log start
migration start
migration fail

Disables dirty logging for the vga slot.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: qemu-kvm/kvm-all.c
===================================================================
--- qemu-kvm.orig/kvm-all.c
+++ qemu-kvm/kvm-all.c
@@ -47,6 +47,7 @@ typedef struct KVMSlot
     ram_addr_t phys_offset;
     int slot;
     int flags;
+    int logging_count;
 } KVMSlot;
 
 typedef struct kvm_dirty_log KVMDirtyLog;
@@ -218,20 +219,11 @@ err:
 /*
  * dirty pages logging control
  */
-static int kvm_dirty_pages_log_change(target_phys_addr_t phys_addr,
-                                      ram_addr_t size, int flags, int mask)
+static int kvm_dirty_pages_log_change(KVMSlot *mem, int flags, int mask)
 {
     KVMState *s = kvm_state;
-    KVMSlot *mem = kvm_lookup_matching_slot(s, phys_addr, phys_addr + size);
     int old_flags;
 
-    if (mem == NULL)  {
-            fprintf(stderr, "BUG: %s: invalid parameters " TARGET_FMT_plx "-"
-                    TARGET_FMT_plx "\n", __func__, phys_addr,
-                    (target_phys_addr_t)(phys_addr + size - 1));
-            return -EINVAL;
-    }
-
     old_flags = mem->flags;
 
     flags = (mem->flags & ~mask) | flags;
@@ -250,16 +242,42 @@ static int kvm_dirty_pages_log_change(ta
 
 int kvm_log_start(target_phys_addr_t phys_addr, ram_addr_t size)
 {
-        return kvm_dirty_pages_log_change(phys_addr, size,
-                                          KVM_MEM_LOG_DIRTY_PAGES,
-                                          KVM_MEM_LOG_DIRTY_PAGES);
+    KVMState *s = kvm_state;
+    KVMSlot *mem = kvm_lookup_matching_slot(s, phys_addr, phys_addr + size);
+
+    if (mem == NULL)  {
+            fprintf(stderr, "BUG: %s: invalid parameters " TARGET_FMT_plx "-"
+                    TARGET_FMT_plx "\n", __func__, phys_addr,
+                    (target_phys_addr_t)(phys_addr + size - 1));
+            return -EINVAL;
+    }
+
+    if (mem->logging_count++)
+        return 0;
+
+    return kvm_dirty_pages_log_change(mem,
+                                      KVM_MEM_LOG_DIRTY_PAGES,
+                                      KVM_MEM_LOG_DIRTY_PAGES);
 }
 
 int kvm_log_stop(target_phys_addr_t phys_addr, ram_addr_t size)
 {
-        return kvm_dirty_pages_log_change(phys_addr, size,
-                                          0,
-                                          KVM_MEM_LOG_DIRTY_PAGES);
+    KVMState *s = kvm_state;
+    KVMSlot *mem = kvm_lookup_matching_slot(s, phys_addr, phys_addr + size);
+
+    if (mem == NULL)  {
+            fprintf(stderr, "BUG: %s: invalid parameters " TARGET_FMT_plx "-"
+                    TARGET_FMT_plx "\n", __func__, phys_addr,
+                    (target_phys_addr_t)(phys_addr + size - 1));
+            return -EINVAL;
+    }
+
+    if (--mem->logging_count)
+        return 0;
+
+    return kvm_dirty_pages_log_change(mem,
+                                      0,
+                                      KVM_MEM_LOG_DIRTY_PAGES);
 }
 
 static int kvm_set_migration_log(int enable)
@@ -273,12 +291,15 @@ static int kvm_set_migration_log(int ena
     for (i = 0; i < ARRAY_SIZE(s->slots); i++) {
         mem = &s->slots[i];
 
-        if (!!(mem->flags & KVM_MEM_LOG_DIRTY_PAGES) == enable) {
-            continue;
-        }
-        err = kvm_set_user_memory_region(s, mem);
-        if (err) {
-            return err;
+        if (mem->memory_size) {
+            if (enable) {
+                err = kvm_log_start(mem->start_addr, mem->memory_size);
+            } else {
+                err = kvm_log_stop(mem->start_addr, mem->memory_size);
+            }
+            if (err) {
+                return err;
+            }
         }
     }
     return 0;
@@ -442,6 +463,7 @@ static void kvm_set_phys_mem(target_phys
 
         /* unregister the overlapping slot */
         mem->memory_size = 0;
+        mem->logging_count = 0;
         err = kvm_set_user_memory_region(s, mem);
         if (err) {
             fprintf(stderr, "%s: error unregistering overlapping slot: %s\n",