migration: simplify error reporting after channel read

migration: simplify error reporting after channel read

The code handling the return value of qio_channel_read proceses
len == 0 (EOF) separately from len < 1  (error), but in both
cases ends up calling qemu_file_set_error_obj() with -EIO as the
errno. This logic can be merged into one codepath to simplify it.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/r/20250801170212.54409-2-berrange@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>