KVM: TDX: Add method to ignore guest instruction emulation

Skip instruction emulation and let the TDX guest retry for MMIO emulation
after installing the MMIO SPTE with suppress #VE bit cleared.

TDX protects TDX guest state from VMM, instructions in guest memory cannot
be emulated.  MMIO emulation is the only case that triggers the instruction
emulation code path for TDX guest.

The MMIO emulation handling flow as following:
- The TDX guest issues a vMMIO instruction. (The GPA must be shared and is
  not covered by KVM memory slot.)
- The default SPTE entry for shared-EPT by KVM has suppress #VE bit set. So
  EPT violation causes TD exit to KVM.
- Trigger KVM page fault handler and install a new SPTE with suppress #VE
  bit cleared.
- Skip instruction emulation and return X86EMU_RETRY_INSTR to let the vCPU
  retry.
- TDX guest re-executes the vMMIO instruction.
- TDX guest gets #VE because KVM has cleared #VE suppress bit.
- TDX guest #VE handler converts MMIO into TDG.VP.VMCALL<MMIO>

Return X86EMU_RETRY_INSTR in the callback check_emulate_instruction() for
TDX guests to retry the MMIO instruction.  Also, the instruction emulation
handling will be skipped, so that the callback check_intercept() will never
be called for TDX guest.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250227012021.1778144-14-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>