x86/hvm: hypercall adjustments

x86/hvm: hypercall adjustments

- share more code between 32- and 64-bit variants
- properly handle continuations for 32-bit guests on 64-bit hv
- properly handle preemption (this must *not* rely on regs->eip, as
- other code may overwrite the value there by calling
- hvm_store_cpu_guest_regs()
- deny hypercall access when called from guest in vm86 mode, which
  requires that ???_guest_x86_mode() make real and vm86 modes distinguishable

Signed-off-by: Jan Beulich <jbeulich@novell.com>