KVM: Avoid corrupting tr in real mode

The real mode tr needs to be set to a specific tss so that I/O
instructions can function.  Divert the new tr values to the real
mode save area from where they will be restored on transition to
protected mode.

This fixes some crashes on reboot when the bios accesses an I/O
instruction.

Signed-off-by: Avi Kivity <avi@qumranet.com>