Fix the implementation of rdtsc in src/mevent_test.c:68

[wierton]

In inline assembly of xhyve/src/mevent_test.c:68, an unused inline
assembly __asm__ __volatile__ ("cpuid") is written here. cpuid is an
instruction which overrides register eax, ebx, ecx and edx by the
information of cpu, (details can be found in x86_64 specification).
This inline assembly is not only unuseful, under some context, it may
corrupt the normal C data flow, and cause errors in C world. For example:

static __inline uint64_t rdtsc(void)
{
	unsigned a, d;
	__asm__ __volatile__ ("cpuid");
	__asm__ __volatile__ ("rdtsc" : "=a" (a), "=d" (d));

	return (((uint64_t) a) | (((uint64_t) d) << 32));
}

int __attribute__((noinline)) set_val(int v) { return v; }

int main() {
  int a = set_val(0);
  int b = set_val(1);
  int c = set_val(2);
  a = a + b; b = b + c; c = c + a;
  uint64_t d = rdtsc();
  printf("%d, %d, %d, %lx\n", a, b, c, d);
  return 0;
}

The above code on my pc (gcc-8 -O2) will output 1, 2, 3, 7bcbf4733e902,
(assume rdtsc now is 7bcbf4733e902), but the actual output should be 1, 3,
3, 7bcbf4733e902, the reason is that cpuid override some registers
unexpectedly.