[GSoC] Making vkernels checkpointable - week 10

Hello,

Last week I was working on restoring vmspaces of virtual processes in
order to be able to run them after vkernel is restored from a
checkpoint. Currently, vmspaces are simply recreated using
cpu_vmspace_alloc() function and the correct one is chosen by calling
vmspace_ctl(). There are still some issues that have to be addressed
but virtual processes continue executing after restoring the vkernel
from a checkpoint.

I also did some work necessary to get vkernel interrupts working. When
restoring a program from a checkpoint flags of every open file are
properly reset (so that kqueue used by vkernels have O_ASYNC set).
Also, vkernel checkpoint signal handler sets kqueue owner the vkernel
new PID.

Paweł