Add 'none' as type for drive's if option

Signed-off-by: Craig Jellick <craig@rancher.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -604,7 +604,7 @@ Special files such as iSCSI devices can be specified using protocol
 specific URLs. See the section for "Device URL Syntax" for more information.
 @item if=@var{interface}
 This option defines on which type on interface the drive is connected.
-Available types are: ide, scsi, sd, mtd, floppy, pflash, virtio.
+Available types are: ide, scsi, sd, mtd, floppy, pflash, virtio, none.
 @item bus=@var{bus},unit=@var{unit}
 These options define where is connected the drive by defining the bus number and
 the unit id.