Update MIPS KVM entry code to support VZ:

 - We need to set GuestCtl0.GM while in guest mode.

 - For cores supporting GuestID, we need to set the root GuestID to
   match the main GuestID while in guest mode so that the root TLB
   refill handler writes the correct GuestID into the TLB.

 - For cores without GuestID where the root ASID dealiases RVA/GPA
   mappings, we need to load that ASID from the gpa_mm rather than the
   per-VCPU guest_kernel_mm or guest_user_mm, since the root TLB maps
   guest physical addresses. We also need to restore the normal process
   ASID on exit.

 - The normal linux process pgd needs restoring on exit, as we can't
   leave the GPA mappings active for kernel code.

 - GuestCtl0 needs saving on exit for the GExcCode field, as it may be
   clobbered if a preemption occurs.

We also need to move the TLB refill handler to the XTLB vector at offset
0x80 on 64-bit VZ kernels, as hardware will use Root.Status.KX to
determine whether a TLB refill or XTLB Refill exception is to be taken
on a root TLB miss from guest mode, and KX needs to be set for kernel
code to be able to access the 64-bit segments.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org