Specify that RXTX_factor is only available for XEN or NSX systems

This patch add a statement to clarify that RXTX_factor parameter
is only available for XEN or NSX based systems.

Change-Id: I500792796b8f14aab1738b049566c32c406ed1e5
Closes-bug: #1339764
Merge "Specify that RXTX_factor is only available for XEN or NSX systems"