cpufreq: xen is default cpufreq, userspace is default governor (override on cmdline)

cpufreq: xen is default cpufreq, userspace is default governor (override on cmdline)

Set userspace governor as default, which stays same effect
as when cpufreq in xen is not enabled. As a result, enable cpufreq
in xen by default to avoid reboot to activate cpufreq. Now it's
always on but w/o performance impact if user doesn't attempt
to change governor.

Add governor option at cmdline, add some warning info for debug.

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>