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

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>