Use virtual 8086 mode for VMX guests with CR0.PE == 0

Use virtual 8086 mode for VMX guests with CR0.PE == 0

When a VMX guest tries to enter real mode, put it in virtual 8086 mode
instead, if that's possible.  Handle all errors and corner cases by
falling back to the real-mode emulator.

This is similar to the old VMXASSIST system except it uses Xen's
x86_emulate emulator instead of having a partial emulator in the guest
firmware.  It more than doubles the speed of real-mode operation on
VMX.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>