INSTALL.DPDK: Update documentation to indicate VFIO support

INSTALL.DPDK: Update documentation to indicate VFIO support

Since DPDK 1.7, VFIO is supported in place of UIO. This allows
a user to avoid having to insert a non-standard kernel module.

This patch updates the documentation with instructions for
setting up OVS with VFIO. As part of this work, VFIO was also
successfully tested with OVS and the DPDK netdev.

[tgraf: Added some more markdown formatting]

Signed-off-by: Mark D. Gray <mark.d.gray@intel.com>
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>