nvmetcli: fix Port and Host operations in shell

Operations against Port and Host entities are broken. When performing
operations like the follow:

/ports> create 1
/hosts> create 1

nvmetcli fails, saying the object doesn't have the attribute we're
trying to access, for instance:
'module' object has no attribute 'Port'

This patch fixes this issue by importing the classes Port and Host in
nvmet/__init__.py which was forgotten during the follow change:

a21ebd54 ("nvmetcli: Fix nvmet import").

Tested-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Tested-by: Sagi Grimberg <sagi@grimberg.m>
Signed-off-by: Mauro S. M. Rodrigues <maurosr@linux.vnet.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

nvmet/__init__.py

-from .nvme import Root, Subsystem, Namespace
+from .nvme import Root, Subsystem, Namespace, Port, Host