tools/misc: Tweak reserved bit handling for xen-cpuid

tools/misc: Tweak reserved bit handling for xen-cpuid

Instead of printing REZ, use NULL pointers to indicate missing information,
and have dump_leaf() print out the bit which is unknown.

E.g.

....
Dynamic sets:
Raw                       178bfbff:fed8320b:2fd3fbff:35c233ff:0000000f:209c01a9:00000000:00006799:00001007:00000000
  [00] 0x00000001.edx     fpu vme de pse tsc msr pae mce cx8 apic sysenter mtrr pge  ...
  [01] 0x00000001.ecx     sse3 pclmulqdq monitor ssse3 fma cx16 sse41 sse42 movebe   ...
  [02] 0x80000001.edx     fpu vme de pse tsc msr pae mce cx8 apic syscall mtrr pge   ...
  [03] 0x80000001.ecx     lahf_lm cmp svm extapic cr8d lzcnt sse4a msse 3dnowpf osvw ...
  [04] 0x0000000d:1.eax   xsaveopt xsavec xgetbv1 xsaves
  [05] 0x00000007:0.ebx   fsgsbase bmi1 avx2 smep bmi2 rdseed adx smap clflushopt sha
  [06] 0x00000007:0.ecx
  [07] 0x80000007.edx     <0> <3> <4> <7> itsc <9> efro <13> <14>
  [08] 0x80000008.ebx     clzero <1> <2> ibpb
  [09] 0x00000007:0.edx
...

which is the output on an AMD EPYC system, where Xen doesn't know about, and
has therefore masked, most of the more advanced thermal/perf features.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>