qemu_coroutine_enter() is now the only user of coroutine_swap(). Both
functions are short, so inline it.

Also, using COROUTINE_YIELD is now even more confusing because this code
is never called during qemu_coroutine_yield() any more. In fact, this
value is never read back, so we can just introduce a new COROUTINE_ENTER
which documents the purpose of the task switch better.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>