i386: Fix GCC warning with snprintf when HAX is enabled

i386: Fix GCC warning with snprintf when HAX is enabled

When HAX is enabled (--enable-hax), GCC 9.2.1 reports issues with
snprintf(). Replacing old snprintf() by g_strdup_printf() fixes the
problem with boundary checks of vm_id and vcpu_id and finally the
warnings produced by GCC.

For more details, one example of warning:
  CC      i386-softmmu/target/i386/hax-posix.o
qemu/target/i386/hax-posix.c: In function ‘hax_host_open_vm’:
qemu/target/i386/hax-posix.c:124:56: error: ‘%02d’ directive output may be
truncated writing between 2 and 11 bytes into a region of size 3
[-Werror=format-truncation=]
  124 |     snprintf(name, sizeof HAX_VM_DEVFS, "/dev/hax_vm/vm%02d", vm_id);
      |                                                        ^~~~
qemu/target/i386/hax-posix.c:124:41: note: directive argument in the range
[-2147483648, 64]
  124 |     snprintf(name, sizeof HAX_VM_DEVFS, "/dev/hax_vm/vm%02d", vm_id);
      |                                         ^~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/stdio.h:867,
                 from qemu/include/qemu/osdep.h:99,
                 from qemu/target/i386/hax-posix.c:14:
/usr/include/bits/stdio2.h:67:10: note: ‘__builtin___snprintf_chk’ output
between 17 and 26 bytes into a destination of size 17
   67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   68 |        __bos (__s), __fmt, __va_arg_pack ());
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>