base: net: Validate descriptor passed to UnixSeqpacketListener
base: net: Validate descriptor passed to UnixSeqpacketListener

The existing file descriptor must refer to a listening socket.

TEST=cargo test, added a new unit test
BUG=b:238931628

Change-Id: Ia6bd5d6d30af69fb4b5fa00865f5528f19f569d9
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3932714
Commit-Queue: Keir Fraser <8e073e04bd9691eaa20733017be556c7bd381775@google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm