perf kvm stat report: Save pid string in opts.target.pid

The 'perf kvm stat report' command uses the kvm->pid_str field to keep
the value of the --pid option. Let's use kvm->opts.target.pid instead.

Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Acked-by: David Ahern <dsahern@gmail.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1409579095-12963-2-git-send-email-yarygin@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>