Initialize state_entry_time to zero for all idle vcpus

Initialize state_entry_time to zero for all idle vcpus

NOW() is not usable since xen time sub-system hasn't
been initialized yet. On my box, it gives a initial
stamp ~60s due to local tsc stamp as zero and TSC
count is started from power on. Then a negative value
is added to runstate of that idle vcpu at schedule
point. The net effect is for some tool like xenpm
to show a big idle time gap between BSP and other APs.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>