qemu: Update asyncOwnerAPI when entering async job phase

qemu: Update asyncOwnerAPI when entering async job phase

In case an async job spans multiple APIs (e.g., incoming migration) the
API that started the job is recorded as the asyncOwnerAPI even though it
is no longer running and the owner thread is updated properly to the one
currently handling the job. Let's also update asyncOwnerAPI to make it
more obvious which is the current (or the most recent) API involved in
the job.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>