If you look at the platforms/unix/vm-display-X11/sqUnixX11.c code you will see that shift tab is handled IFF
#if defined(XK_ISO_Left_Tab)
is true. That comes from keysymdef.h which looks to be (on my pi, at least) defined in /usr/include/X11/keysymdef.h Thus the Pi understands shift-tab. If your unix machine/vm combo does not, then find out where the analogue of keysymdefs.h lives and see why XK_ISO_Left_Tab is not defined.

tim
--
tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
MIPS:  Meaningless Indicator of Processor Speed.