doc: consolidate VFIO content on Linux drivers page

doc: consolidate VFIO content on Linux drivers page

Rather than having separate sections for VFIO and VFIO no-iommu mode, as
well as a separate section further down the document on troubleshooting
VFIO, we can consolidate all these as subsections into a primary VFIO
section. This section starts with the basics of VFIO use, then covers
no-iommu mode, before moving on to the more advanced topics such as
creating VFs and ending with the troubleshooting subsection.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>