vl.c: fix migration failure for 3.1 and older machine types

Migration from QEMU(v4.0) fails when using 3.1 or older machine
type. For example if one attempts to migrate
QEMU-2.12 started as
  qemu-system-ppc64 -nodefaults -M pseries-2.12 -m 4096 -mem-path /tmp/
to current master, it will fail with
  qemu-system-ppc64: Unknown ramblock "ppc_spapr.ram", cannot accept migration
  qemu-system-ppc64: error while loading state for instance 0x0 of device 'ram'
  qemu-system-ppc64: load of migration failed: Invalid argument

Caused by 900c0ba3 commit which switches main RAM allocation to
memory backends and the fact in 3.1 and older QEMU, backends used
full[***] QOM path as memory region name instead of backend's name.
That was changed after 3.1 to use prefix-less names by default
(fa0cb34d) for new machine types.
*** effectively makes main RAM memory region names defined by
MachineClass::default_ram_id being altered with '/objects/' prefix
and therefore migration fails as old QEMU sends prefix-less
name while new QEMU expects name with prefix when using 3.1 and
older machine types.

Fix it by forcing implicit[1] memory backend to always use
prefix-less names for its memory region by setting
  'x-use-canonical-path-for-ramblock-id'
property to false.

1) i.e. memory backend created by compat glue which maps
-m/-mem-path/-mem-prealloc/default RAM size into
appropriate backend type/options to match old CLI format.

Fixes: 900c0ba3Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reported-by: NLukáš Doktor <ldoktor@redhat.com>
Message-Id: <20200304172748.15338-1-imammedo@redhat.com>
Tested-by: NLukáš Doktor <ldoktor@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

softmmu/vl.c

@@ -2801,6 +2801,9 @@ static void create_default_memdev(MachineState *ms, const char *path)
     object_property_set_int(obj, ms->ram_size, "size", &error_fatal);
     object_property_add_child(object_get_objects_root(), mc->default_ram_id,
                               obj, &error_fatal);
+    /* Ensure backend's memory region name is equal to mc->default_ram_id */
+    object_property_set_bool(obj, false, "x-use-canonical-path-for-ramblock-id",
+                             &error_fatal);
     user_creatable_complete(USER_CREATABLE(obj), &error_fatal);
     object_unref(obj);
     object_property_set_str(OBJECT(ms), mc->default_ram_id, "memory-backend",