net/socket: Don't treat odd socket type as SOCK_STREAM

In net_socket_fd_init(), the 'default' case is odd: it warns,
then continues as if the socket type was SOCK_STREAM. The
comment explains "this could be a eg. a pty", but that makes
no sense. If @fd really was a pty, getsockopt() would fail
with ENOTSOCK. If @fd was a socket, but neither SOCK_DGRAM nor
SOCK_STREAM. It should not be treated as if it was SOCK_STREAM.

Turn this case into an Error. If there is a genuine reason to
support something like SOCK_RAW, it should be explicitly
handled.

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