x86/hvm: Disallow the creation of HVM domains without Local APIC emulation

x86/hvm: Disallow the creation of HVM domains without Local APIC emulation

There are multiple problems, not necesserily limited to:

 * Guests which configure event channels via hvmop_set_evtchn_upcall_vector(),
   or which hit %cr8 emulation will cause Xen to fall over a NULL vlapic->regs
   pointer.

 * On Intel hardware, disabling the TPR_SHADOW execution control without
   reenabling CR8_{LOAD,STORE} interception means that the guests %cr8
   accesses interact with the real TPR.  Amongst other things, setting the
   real TPR to 0xf blocks even IPIs from interrupting this CPU.

 * On hardware which sets up the use of Interrupt Posting, including
   IOMMU-Posting, guests run without the appropriate non-root configuration,
   which at a minimum will result in dropped interrupts.

Whether no-LAPIC mode is of any use at all remains to be seen.

This is XSA-256.

Reported-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>