nvmetcli: Correct xrange usage for py3

nvmetcli: Correct xrange usage for py3

If you are in a namespace and simply do a 'create' without specifying
a value you will get:

/subsystems/n...f8/namespaces> create
name 'xrange' is not defined
subsystems/n...f8/namespaces>

This is because xrange is not defined in python3 as python3 changed
it to range.  As the code is already using six use six.move.xrange
which works for both python2 & python3.

Signed-off-by: Tony Asleson <tasleson@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>