perf evsel: Display 0x for hex values when printing the attribute

Need to display '0x' prefix for hex values otherwise it is not obvious
they are hex.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1434027064-7554-1-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>