nvme-fabrics: generate spec-compliant UUID NQNs

The default host NQN, which is generated based on the host's UUID,
does not follow the UUID-based NQN format laid out in the NVMe 1.3
specification.  Remove the "NVMf:" portion of the NQN to match the spec.

Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>