coroutine: Introduce qemu_coroutine_enter_if_inactive()

coroutine: Introduce qemu_coroutine_enter_if_inactive()

In the context of asynchronous work, if we have a worker coroutine that
didn't yield, the parent coroutine cannot be reentered because it hasn't
yielded yet. In this case we don't even have to reenter the parent
because it will see that the work is already done and won't even yield.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>