Ensure client is marked for close in all error paths

Ensure client is marked for close in all error paths

Currently if the keepalive timer triggers, the 'markClose'
flag is set on the virNetClient. A controlled shutdown will
then be performed. If an I/O error occurs during read or
write of the connection an error is raised back to the
caller, but the connection isn't marked for close. This
patch ensures that all I/O error scenarios always result
in the connection being marked for close.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>