nvmetcli: fix Referral operation in shell

It's not possible to create or list a Referral under Port entity,
because Referral class is not imported and thus it fails:

/ports/1/referrals> ls

returning "'module' object has no attribute 'Referral'".

This is caused by changes introduced in:
a21ebd54 ("nvmetcli: Fix nvmet import").

This patch also fixes the attribute type helper expected for Referral's
portid attribute. It was 'int', causing the system to show error
messages like:

"'UIReferralNode' object has no attribute 'ui_type_int'".
The correct type is 'number'.

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