vfio/pci: Make IGD support a configurable option

Allow the code which provides extensions to support direct assignment
of Intel IGD (GVT-d) to be compiled out of the kernel if desired.  The
config option for this was previously automatically enabled on X86,
therefore the default remains Y.  This simply provides the option to
disable it even for X86.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/Kconfig

@@ -28,5 +28,13 @@ config VFIO_PCI_INTX
 	def_bool y if !S390
 
 config VFIO_PCI_IGD
-	depends on VFIO_PCI
-	def_bool y if X86
+	bool "VFIO PCI extensions for Intel graphics (GVT-d)"
+	depends on VFIO_PCI && X86
+	default y
+	help
+	  Support for Intel IGD specific extensions to enable direct
+	  assignment to virtual machines.  This includes exposing an IGD
+	  specific firmware table and read-only copies of the host bridge
+	  and LPC bridge config space.
+
+	  To enable Intel IGD assignment through vfio-pci, say Y.