perf tools: Let default config be defined for a PMU

perf tools: Let default config be defined for a PMU

This allows default config terms to be provided for a PMU. So, for
example, when the Intel PT PMU is added, it will be possible to specify:

	intel_pt//

which will be the same as:

	intel_pt/tsc=1,noretcomp=0/

meaning that the trace should contain TSC timestamps and perform 'return
compression'.

An important consideration of this patch is that it must be possible to
overwrite the default values.  That has meant changing the logic so that
a zero value can replace a non-zero value.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1406786474-9306-7-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>