tools/libxg: Fix uninitialised variable in write_x86_cpu_policy_records()

tools/libxg: Fix uninitialised variable in write_x86_cpu_policy_records()

Various version of gcc, when compiling with -Og, complain:

  xg_sr_common_x86.c: In function 'write_x86_cpu_policy_records':
  xg_sr_common_x86.c:92:12: error: 'rc' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     92 |     return rc;
        |            ^~

The complaint is legitimate, and can occur with unexpected behaviour of two
related hypercalls in combination with a libc which permits zero-length
malloc()s.

Have an explicit rc = 0 on the success path, and make the MSRs record error
handling consistent with the CPUID record before it.

Fixes: f6b2b8ec53d ("libxc/save: Write X86_{CPUID,MSR}_DATA records")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>