perf kvm: Use NSEC_PER_USEC

perf kvm: Use NSEC_PER_USEC

Following kernel practices and better documenting units of time.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hemant Kumar <hemant@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-5x6p6fmzrogonpbnkkkw4usk@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>