timeval: Initialize 'unix_epoch' for Windows correctly.
Till now, we were initializing 'unix_epoch' through time_init().
But if there was a call directly to xgettimeofday(), we would
miss the initialization causing overflows. This commit fixes it
by pre-calculating the value and assigning it globally.
Also add-in a missing return statement.
Reported-by: Alessandro Pilotti <apilotti@cloudbasesolutions.com> Reported-by: Linda Sun <lsun@vmware.com> Signed-off-by: Gurucharan Shetty <gshetty@nicira.com> Acked-by: Ben Pfaff <blp@nicira.com>