net/socket: Convert several helper functions to Error

Currently, net_socket_mcast_create(), net_socket_fd_init_dgram() and
net_socket_fd_init() use the function such as fprintf(), perror() to
report an error message.

Now, convert these functions to Error.

Cc: jasowang@redhat.com
Cc: armbru@redhat.com
Cc: berrange@redhat.com
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>