memory: count number of active VGA logging clients

For a board that has multiple framebuffer devices, both of them
might want to use DIRTY_MEMORY_VGA on the same memory region.
The lack of reference counting in memory_region_set_log makes
this very awkward to implement.
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/exec/memory.h

@@ -180,6 +180,7 @@ struct MemoryRegion {
     bool warning_printed; /* For reservations */
     bool flush_coalesced_mmio;
     bool global_locking;
+    uint8_t vga_logging_count;
     MemoryRegion *alias;
     hwaddr alias_offset;
     int32_t priority;

memory.c

@@ -1433,8 +1433,15 @@ void memory_region_notify_iommu(MemoryRegion *mr,
 void memory_region_set_log(MemoryRegion *mr, bool log, unsigned client)
 {
     uint8_t mask = 1 << client;
+    uint8_t old_logging;
 
     assert(client == DIRTY_MEMORY_VGA);
+    old_logging = mr->vga_logging_count;
+    mr->vga_logging_count += log ? 1 : -1;
+    if (!!old_logging == !!mr->vga_logging_count) {
+        return;
+    }
+
     memory_region_transaction_begin();
     mr->dirty_log_mask = (mr->dirty_log_mask & ~mask) | (log * mask);
     memory_region_update_pending |= mr->enabled;