libqtest: add qtest_socket_server()

libqtest: add qtest_socket_server()

Add an API that returns a new UNIX domain socket in the listen state.
The code for this was already there but only used internally in
init_socket().

This new API will be used by vhost-user-blk-test.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210223144653.811468-3-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>