memory: add ref/unref calls

Add ref/unref calls at the following places:

- places where memory regions are stashed by a listener and
  used outside the BQL (including in Xen or KVM).

- memory_region_find callsites

- creation of aliases and containers (only the aliased/contained
  region gets a reference to avoid loops)

- around calls to del_subregion/add_subregion, where the region
  could disappear after the first call

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/hw/virtio/dataplane/hostmem.h

@@ -18,6 +18,7 @@
 #include "qemu/thread.h"
 
 typedef struct {
+    MemoryRegion *mr;
     void *host_addr;
     hwaddr guest_addr;
     uint64_t size;