Using error_prepend() in qio_channel_readv_full_all() causes a segfault
as errp is not set when ret is 0. This results in the failure of iotest
83. Replacing with error_setg() fixes the problem.

Additionally, removes a full stop at the end of error message

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Fixes: bebab91e
  (io: add qio_channel_readv_full_all_eof & qio_channel_readv_full_all helpers)
Message-Id: <be476bcdb99e820fec0fa09fe8f04c9dd3e62473.1613128220.git.jag.raman@oracle.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>