vlib: fix initial stats time for the process nodes

vlib: fix initial stats time for the process nodes

Uninitialized stats time for new dynamic processes (like unix
cli) leads to invalid and too high clocks diff, breaking the
stats collecting.

  $ sudo vppctl sh runtime | grep unix
  unix-cli-new-session  any wait      0  0  30   2.72e3  0.00
  unix-cli-process-0      active     14  0  27  3.73e15  0.00
  unix-epoll-input       polling   6464  0   0   3.56e6  0.00
  unix-epoll-input       polling  68360  0   0   1.31e3  0.00

Type: fix
Signed-off-by: Vladislav Grishenko <[email protected]>
Change-Id: Ic70678b67a3fb3f78ed4a1a03077ae243ed5d7cd