sockets: strengthen test suite IP protocol availability checks

sockets: strengthen test suite IP protocol availability checks

Instead of just checking whether it is possible to bind() on a socket, also
check that we can successfully connect() to the socket we bound to. This
more closely replicates the level of functionality that tests will actually
use.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>