test/vsock: add ioctl unsent bytes test

Introduce two tests, one for SOCK_STREAM and one for SOCK_SEQPACKET,
which use SIOCOUTQ ioctl to check that the number of unsent bytes is
zero after delivering a packet.

vsock_connect and vsock_accept are no longer static: this is to
create more generic tests, allowing code to be reused for SEQPACKET
and STREAM.

Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>