[IA64] vti fault handler clean up: make VMX_DO_SAVE_MIN aware of fault in xen

[IA64] vti fault handler clean up: make VMX_DO_SAVE_MIN aware of fault in xen

Current VMX_DO_SAVE_MIN is simplified from DO_SAVE_MIN assuming
that the faults are in VTi guest. Discard the assumption and make
code more generic.  Make it able to handle faults in both guest and xen.
Set pKStk and pUStk in VMX_DO_SAVE_MIN and use them to get the correct
place to save registers so that don't overwrite guest mode register
in vmx fault handler.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>