The code in builtin-kvm.c to generate filename for perf-kvm is useful to
other command such as builtin-diff.

This patch move the related code form builtin-kvm.c to util/util.c and
wrap them in a function named get_filename_for_perf_kvm.
Signed-off-by: NDongsheng Yang <yangds.fnst@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/5e09a5c47e8a495e888cbdc65a6fafb2c950f529.1386368672.git.yangds.fnst@cn.fujitsu.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>