Add a caption to the figure describing the Real-Time Linux VM running
on ACRN. Without this text, the HTML output does not add the Figure
number below it. That figure number is still used in the paragraph
making it hard for the reader to understand which figure is really
referenced (except by clicking on the hyperlink)
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>