x86: don't drop guest visible state updates when 64-bit PV guest is in user mode

x86: don't drop guest visible state updates when 64-bit PV guest is in user mode

Since 64-bit PV uses separate kernel and user mode page tables, kernel
addresses (as usually provided via VCPUOP_register_runstate_memory_area
and possibly via VCPUOP_register_vcpu_time_memory_area) aren't
necessarily accessible when the respective updating occurs. Add logic
for toggle_guest_mode() to take care of this (if necessary) the next
time the vCPU switches to kernel mode.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>