KVM: x86: clear stale x86_emulate_ctxt->intercept value

After commit 07721fee ("KVM: nVMX: Don't emulate instructions in guest
mode") Hyper-V guests on KVM stopped booting with:

 kvm_nested_vmexit:    rip fffff802987d6169 reason EPT_VIOLATION info1 181
    info2 0 int_info 0 int_info_err 0
 kvm_page_fault:       address febd0000 error_code 181
 kvm_emulate_insn:     0:fffff802987d6169: f3 a5
 kvm_emulate_insn:     0:fffff802987d6169: f3 a5 FAIL
 kvm_inj_exception:    #UD (0x0)

"f3 a5" is a "rep movsw" instruction, which should not be intercepted
at all.  Commit c44b4c6a ("KVM: emulate: clean up initializations in
init_decode_cache") reduced the number of fields cleared by
init_decode_cache() claiming that they are being cleared elsewhere,
'intercept', however, is left uncleared if the instruction does not have
any of the "slow path" flags (NotImpl, Stack, Op3264, Sse, Mmx, CheckPerm,
NearBranch, No16 and of course Intercept itself).

Fixes: c44b4c6a ("KVM: emulate: clean up initializations in init_decode_cache")
Fixes: 07721fee ("KVM: nVMX: Don't emulate instructions in guest mode")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -5173,6 +5173,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	ctxt->fetch.ptr = ctxt->fetch.data;
 	ctxt->fetch.end = ctxt->fetch.data + insn_len;
 	ctxt->opcode_len = 1;
+	ctxt->intercept = x86_intercept_none;
 	if (insn_len > 0)
 		memcpy(ctxt->fetch.data, insn, insn_len);
 	else {