vfio: header for build support

vfio: header for build support

Add VFIO compilation option to linuxapp config.

Adding a header that will determine if VFIO support should be compiled
in. If VFIO is enabled in config (and it's enabled by default), then the
header will also check for kernel version. If VFIO is enabled in config
and if the kernel version is 3.6+, then VFIO_PRESENT will be defined.
This is the macro that should be used to determine if VFIO support is
being compiled in.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: HuilongX Xu <huilongx.xu@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>