This fixes a flaw pointed out in review of PR kata-containers#8185.  Creation of the
directory semantically fits better into VM preparation than VM launch.

Signed-off-by: Pavel Mores <pmores@redhat.com>

We'll want to capture qemu's stderr in parallel with normal runtime-rs
execution.  Tokio's primitives make this much easier than std's.  This
also makes child process management more consistent across runtime-rs
(i.e. virtiofsd child process is already launched and managed using tokio).

Some changes were necessary due to tokio functions being slightly different
from their std counterparts.  Child::kill() is now async and Child::id()
now returns an Option.

Signed-off-by: Pavel Mores <pmores@redhat.com>

Qemu stderr monitoring runs in its own asynchronous green thread.
For that, `stderr` is taken out of the Child representing the qemu child
process to avoid partial move and make it possible for the main thread
still to call functions on QemuInner::qemu_process (e.g. kill(), id()).

Fixes kata-containers#8937

Signed-off-by: Pavel Mores <pmores@redhat.com>