I've been getting a "Failed VMEntry" when trying to boot a second
VMX guest (while the first one is still running, but is no longer in
real mode).  This patch fixes it.

VMX(assist) uses vm86 in its real mode emulation.  Upon vmentry
into a guest in vm86 mode, all segment bases must be equal to the
corresponding segment selector shifted left four bits.  The vmx
routine vmx_load_cpu_guest_regs() was loading the segment
selectors.  Now it makes sure to set the segment bases appropriately
if we're in vm86 mode.

Tested on 64bit hypervisor with 2 64-bit VMX domUs (on
2-way dom0).

Dave