io: Add helper for setting socket send buffer size

io: Add helper for setting socket send buffer size

Testing reading and writing from qemu-nbd using a unix domain socket
shows that the platform default send buffer size is too low, leading to
poor performance and hight cpu usage.

Add a helper for setting socket send buffer size to be used in NBD code.
It can also be used in other contexts.

We don't need a helper for receive buffer size since it is not used with
unix domain sockets. This is documented for Linux, and not documented
for macOS.

Failing to set the socket buffer size is not a fatal error, but the
caller may want to warn about the failure.

Signed-off-by: Nir Soffer <nirsof@gmail.com>
Message-ID: <20250517201154.88456-2-nirsof@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>