sockets: Fix socket_sockaddr_to_address_unix() for abstract sockets

sockets: Fix socket_sockaddr_to_address_unix() for abstract sockets

Commit 776b97d360 "qemu-sockets: add abstract UNIX domain socket
support" neglected to update socket_sockaddr_to_address_unix().  The
function returns a non-abstract socket address for abstract
sockets (wrong) with a null @path (also wrong; a non-optional QAPI str
member must never be null).

The null @path is due to confused code going back all the way to
commit 17c55decec "sockets: add helpers for creating SocketAddress
from a socket".

Add the required special case, and simplify the confused code.

Fixes: 776b97d3605ed0fc94443048fdf988c7725e38a9
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>