fence_rhevm fixing help for plug param

The help for the fence_rhevm -n (--plug) parameter is incorrect.

The current help is :

-n, --plug=[id] Physical plug number on device, UUID or

identification of machine

should be

-n, --plug=[name] The VM name in RHV

The fence_rhevm using the VM name passed in order to get the UUID of
that VM such that the VM can be stopped/started user REST API

<url>/vms/<vm uuid>/stop or <url>/vms/<vm uuid>/start

Issue : https://github.com/ClusterLabs/fence-agents/issues/224
Signed-off-by: emesika <emesika@redhat.com>