io/channel-socket: rework qio_channel_socket_copy_fds()

io/channel-socket: rework qio_channel_socket_copy_fds()

We want to switch from qemu_socket_set_block() to newer
qemu_set_blocking(), which provides return status of operation,
to handle errors.

Still, we want to keep qio_channel_socket_readv() interface clean,
as currently it allocate @fds only on success.

So, in case of error, we should close all incoming fds and keep
user's @fds untouched or zero.

Let's make separate functions qio_channel_handle_fds() and
qio_channel_cleanup_fds(), to achieve what we want.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>