virtio_fs: add informative log for new tag discovery

Enhance the device probing process by adding a log message when a new
virtio-fs tag is successfully discovered. This improvement provides
better visibility into the initialization of virtio-fs devices.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Message-Id: <20241006184324.8497-1-mgurtovoy@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>