hw/mem/nvdimm: fix error message for 'unarmed' flag

In the ACPI specification [1], the 'unarmed' bit is set when a device
cannot accept a persistent write. This means that when a memdev is
read-only, the 'unarmed' flag must be turned on. The logic is correct,
just changing the error message.

[1] ACPI NFIT NVDIMM Region Mapping Structure "NVDIMM State Flags" Bit 3

Fixes: dbd730e859 ("nvdimm: check -object memory-backend-file, readonly=on option")
Signed-off-by: Julia Suvorova <jusual@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: David Hildenbrand <david@redhat.com>
Message-Id: <20221023195812.15523-1-jusual@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/mem/nvdimm.c

@@ -149,7 +149,7 @@ static void nvdimm_prepare_memory_region(NVDIMMDevice *nvdimm, Error **errp)
     if (!nvdimm->unarmed && memory_region_is_rom(mr)) {
         HostMemoryBackend *hostmem = dimm->hostmem;
 
-        error_setg(errp, "'unarmed' property must be off since memdev %s "
+        error_setg(errp, "'unarmed' property must be 'on' since memdev %s "
                    "is read-only",
                    object_get_canonical_path_component(OBJECT(hostmem)));
         return;