More fix on X modules.
The X server modules (display driver and mouse driver) bundled with
VMware Tools doesn't work with X.Org 7.2 server. So install these
modules if and only if X is actually XFree86 4.x.
Also fix pkg-plist (sorry the last commit makes plist broken.)
Bump vmware-tools6 PORTREVISION to note the changes above.
For X.Org 7.2 users:
x11-drivers/xf86-video-vmware is the video driver for VMs, that
is installed by default.
x11-drivers/xf86-input-vmmouse is the mouse driver for VMs.
However, it doesn't need if vmware-toolbox runs.