test/vsock: MSG_ZEROCOPY flag tests

This adds three tests for MSG_ZEROCOPY feature:
1) SOCK_STREAM tx with different buffers.
2) SOCK_SEQPACKET tx with different buffers.
3) SOCK_STREAM test to read empty error queue of the socket.

Patch also works as preparation for the next patches for tools in this
patchset: vsock_perf and vsock_uring_test:
1) Adds several new functions to util.c - they will be also used by
   vsock_uring_test.
2) Adds two new functions for MSG_ZEROCOPY handling to a new source
   file - such source will be shared between vsock_test, vsock_perf and
   vsock_uring_test, thus avoiding code copy-pasting.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>