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: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo 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;