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>

target-sparc/mmu_helper.c

@@ -845,6 +845,7 @@ hwaddr cpu_get_phys_page_debug(CPUSPARCState *env, target_ulong addr)
         }
     }
     section = memory_region_find(get_system_memory(), phys_addr, 1);
+    memory_region_unref(section.mr);
     if (!int128_nz(section.size)) {
         return -1;
     }