perf kvm: Add arch neutral function to choose event for perf kvm record

'perf kvm record' uses 'cycles'(if the user did not specify any event)
as the default event to profile the guest.

This will not provide any proper samples from the guest incase of
powerpc architecture, since in powerpc the PMUs are controlled by the
guest rather than the host.

Patch adds a function to pick an arch specific event for 'perf kvm
record', instead of selecting 'cycles' as a default event for all
architectures.

For powerpc this function checks for any user specified event, and if
there isn't any it returns invalid instead of proceeding with 'cycles'
event.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lore.kernel.org/lkml/20190718181749.30612-2-anju@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>