sheepdog: Fix crash in co_read_response()

This fixes a regression introduced in commit 9d456654.

aio_co_wake() can only be used to reenter a coroutine that was already
previously entered, otherwise co->ctx is uninitialised and we access
garbage. Using it immediately after qemu_coroutine_create() like in
co_read_response() is wrong and causes segfaults.

Replace the call with aio_co_enter(), which gets an explicit AioContext
parameter and works even for new coroutines.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1491919733-21065-1-git-send-email-kwolf@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
