selftests: kvm: Use a shorter encoding to clear RAX

If debug_regs.c is built with newer binutils, the resulting binary is "optimized"
by the assembler:

asm volatile("ss_start: "
"xor %%rax,%%rax\n\t"
"cpuid\n\t"
"movl $0x1a0,%%ecx\n\t"
"rdmsr\n\t"
: : : "rax", "ecx");

is translated to :

000000000040194e <ss_start>:
40194e: 31 c0 xor %eax,%eax <----- rax->eax?
401950: 0f a2 cpuid
401952: b9 a0 01 00 00 mov $0x1a0,%ecx
401957: 0f 32 rdmsr

As you can see rax is replaced with eax in target binary code.
This causes a difference is the length of xor instruction (2 Byte vs 3 Byte),
and makes the hard-coded instruction length check fail:

/* Instruction lengths starting at ss_start */
int ss_size[4] = {
3, /* xor */ <-------- 2 or 3?
2, /* cpuid */
5, /* mov */
2, /* rdmsr */
};

Encode the shorter version directly and, while at it, fix the "clobbers"
of the asm.

Cc: [e-mail]
Signed-off-by: Yang Weijiang <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

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

···
 
 	/* Single step test, covers 2 basic instructions and 2 emulated */
 	asm volatile("ss_start: "
-		     "xor %%rax,%%rax\n\t"
+		     "xor %%eax,%%eax\n\t"
 		     "cpuid\n\t"
 		     "movl $0x1a0,%%ecx\n\t"
 		     "rdmsr\n\t"
-		     : : : "rax", "ecx");
+		     : : : "eax", "ebx", "ecx", "edx");
 
 	/* DR6.BD test */
 	asm volatile("bd_start: mov %%dr0, %%rax" : : : "rax");