io: Add a QIOChannelSocket cleanup test

This patch adds a test to verify that the QIOChannel framework will not
unlink a filesystem unix socket unless the _FEATURE_LISTEN bit is set.

Due to a bug introduced in 74b6ce43, the framework would unlink the
entry if the _FEATURE_SHUTDOWN bit was set, regardless of the presence
of _FEATURE_LISTEN.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
