webadmin: Improve VM interface explanatory text

Changed the text to conform with the changes to the widget.

Change-Id: I79d0d3d62f48c1d56292e65f5087a7a7100e118a
Bug-Url: https://bugzilla.redhat.com/1032513
Signed-off-by: Lior Vernia <lvernia@redhat.com>