For the container use case, we need to relay the exit code from
userspace in the microVM all the way to the process using libkrun to
launch the VMM.

Since ioctls are passed mostly unmodified from userspace in the guest
to the virtio-fs device, we can use them as a mechanism for transporting
this information without requiring any specific support in the guest's
kernel.

Here we create an AtomicI32 and wire it up between virtio-fs and Vmm,
using it as exit code if userspace has set it to some value other than
i32::MAX. Otherwise, we keep using the vCPU exit code, as we did before.

Signed-off-by: Sergio Lopez <slp@redhat.com>

For consistency for the container isolation use case, let's use in init
the same exit codes as chroot and podman do:

 125: "init" cannot set up the environment inside the microVM.
 126: "init" can find the executable to be run inside the microVM but cannot not execute it.
 127: "init" cannot find the executable to be run inside the microVM.

Co-authored-by: Matej Hrica <30753707+mtjhrc@users.noreply.github.com>
Signed-off-by: Sergio Lopez <slp@redhat.com>