misc: Reserve minor for VFIO

VFIO currently allocates it's own dynamic chardev range, reserving the
first minor for the control part of the interface (/dev/vfio/vfio) and
the remainder for VFIO groups (/dev/vfio/$GROUP).  This works, but it
doesn't support auto loading.  For instance when libvirt checks for
VFIO support it looks for /dev/vfio/vfio, which currently doesn't
exist unless the vfio module is loaded.  By converting the control
device to a misc driver and reserving a static minor, we can enable
auto loading.

Reserving the minor is a prerequist to that conversion.  Minor 196
is unused by anything currently in the kernel.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devices.txt

@@ -409,6 +409,7 @@ Your cooperation is appreciated.
 		193 = /dev/d7s		SPARC 7-segment display
 		194 = /dev/zkshim	Zero-Knowledge network shim control
 		195 = /dev/elographics/e2201	Elographics touchscreen E271-2201
+		196 = /dev/vfio/vfio	VFIO userspace driver interface
 		198 = /dev/sexec	Signed executable interface
 		199 = /dev/scanners/cuecat :CueCat barcode scanner
 		200 = /dev/net/tun	TAP/TUN network device

include/linux/miscdevice.h

@@ -30,6 +30,7 @@
 #define STORE_QUEUE_MINOR	155
 #define I2O_MINOR		166
 #define MICROCODE_MINOR		184
+#define VFIO_MINOR		196
 #define TUN_MINOR		200
 #define CUSE_MINOR		203
 #define MWAVE_MINOR		219	/* ACP/Mwave Modem */