selftests: kvm: Fix assert failure in single-step test

This is a follow-up patch to fix an issue left in commit:
98b0bf0273
selftests: kvm: Use a shorter encoding to clear RAX

With the change in the commit, we also need to modify "xor" instruction
length from 3 to 2 in array ss_size accordingly to pass below check:

for (i = 0; i < (sizeof(ss_size) / sizeof(ss_size[0])); i++) {
        target_rip += ss_size[i];
        CLEAR_DEBUG();
        debug.control = KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP;
        debug.arch.debugreg[7] = 0x00000400;
        APPLY_DEBUG();
        vcpu_run(vm, VCPU_ID);
        TEST_ASSERT(run->exit_reason == KVM_EXIT_DEBUG &&
                    run->debug.arch.exception == DB_VECTOR &&
                    run->debug.arch.pc == target_rip &&
                    run->debug.arch.dr6 == target_dr6,
                    "SINGLE_STEP[%d]: exit %d exception %d rip 0x%llx "
                    "(should be 0x%llx) dr6 0x%llx (should be 0x%llx)",
                    i, run->exit_reason, run->debug.arch.exception,
                    run->debug.arch.pc, target_rip, run->debug.arch.dr6,
                    target_dr6);
}

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Message-Id: <20200826015524.13251-1-weijiang.yang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/x86_64/debug_regs.c

@@ -73,7 +73,7 @@ int main(void)
 	int i;
 	/* Instruction lengths starting at ss_start */
 	int ss_size[4] = {
-		3,		/* xor */
+		2,		/* xor */
 		2,		/* cpuid */
 		5,		/* mov */
 		2,		/* rdmsr */