Update xenserver_vm_create.rb added param -j, --json-attributes

Added ability to pass attributes. Example:
```
knife xenserver vm create --xenserver-host $HOST --xenserver-password $PASS --vm-name $NAME --vm-ip $IP --vm-netmask $MASK --vm-gateway $GATE --vm-dns 8.8.8.8 --vm-template centos-cloudtechlab --keep-template-networks --ssh-password cl0udAdmin ssh-user root --distro chef-full --vm-memory 2048 -r $COOKBOOK::$RECIPE -j '{"creds":{"admin_password":"cl0udAdmin","mysql_password":"cl0udAdmin","ssh_keypair":"openstack"},"ip":{"qpid":"192.168.251.10","keystone":"192.168.251.11","swift":"192.168.251.12","glance":"192.168.251.13","cinder":"192.168.251.14","quantum":"192.168.251.15","nova":"192.168.251.16","heat":"192.168.251.17"}}'
```