virtio-mem: don't warn about THP sizes on a kernel without THP support

virtio-mem: don't warn about THP sizes on a kernel without THP support

If the config directory in sysfs does not exist at all, we are dealing
with a system that does not support THPs. Simply use 1 MiB block size
then, instead of warning "Could not detect THP size, falling back to
..." and falling back to the default THP size.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: Juraj Marcin <jmarcin@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20240910163433.2100295-1-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>