qapi: qga: Clarify when out-data and err-data are populated

qapi: qga: Clarify when out-data and err-data are populated

If output is being captured for a guest-exec invocation, the out-data
and err-data fields of guest-exec-status are only populated after the
process is reaped. This is somewhat counter intuitive and too late to
change. Thus, it would be good to document the behavior.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>