nvmetcli: create json file's dir first if it doesn't exist

The saveconfig operation will be failed as the /etc/nvmet dir
doesn't exist by default, so create it first before save operation.

$ ./nvmetcli
/> saveconfig
[Errno 2] No such file or directory: '/etc/nvmet/config.json.temp'

Signed-off-by: Yi Zhang <yizhan@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>