utils_misc: Add monotonic_time() method

Python 3.3 introduce support for time.monotonic(), which is useful when
measuring times as it will not jump if the time is set. As the timing is
important in some tests, this is very useful for virt-test.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>
Merge pull request #1579 from zqad/monotonic_time

utils_misc: Add monotonic_time() method