vTPM: add warning message about non-persistent data (#598)

* vTPM: add warning message about non-persistent data

* Update source/adminguide/virtual_machines.rst

Co-authored-by: dahn <daan.hoogland@gmail.com>

* Update source/adminguide/virtual_machines.rst

---------

Co-authored-by: dahn <daan.hoogland@gmail.com>

Author: weizhouapache

source/adminguide/virtual_machines.rst

@@ -1047,6 +1047,9 @@ TPM version for KVM. There are two options:
 |vm-settings-virtual-tpm-enabled-vmware.png|
 Enable or disable vTPM for VMware.
 
+.. note::
+     Data stored in the virtual TPM device is not persistent.
+
 Instance Snapshots
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