sched: ignore backward jumps in clock

Due to an unknown bug (bug in our HPET driver?), OSv crashed on our build
machine with an assertion that the clock only goes foward. The clock
jumping backward is a bad sign, but it's not really necessary to crash the
VM when it happens, assuming it only happens rarely. This patch makes the
scheduler handle time<0 just like time==0.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>