Decouple Vmm code into controller/builder and live-vmm

[acatangiu]

No description provided.

[dianpopa]

With a Vmm controller/builder in place, the road to freeing the seccomp filter of the timerfd_create syscall seems a lot simpler since the Vmm Builder could simply just allocate the needed timerfds before starting the microVm. If we aim for a more forward looking solution we could try implementing a resource pooling approach like the one displayed in #1281.