perf, tools: Expand PMU events by prefetch match

When the user specifies a pmu directly, expand it automatically
with a prefix match, similar as we do for the normal aliases now.
This allows to specify attributes for duplicated boxes quickly.
For example uncore_cbox_{0,8}/.../ can be now specified as cbox/.../
and it gets automatically expanded.

Signed-off-by: Andi Kleen <ak@linux.intel.com>