CA-140334: don't complain when tapdisk control socket doesn't exist

tapdisks may be destroyed at any time by the SM, therefore their control
socket might disappear abruptly. This is expected and complaining about
innocuous errors like that complicates debugging.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Yes, if we get an error from connect it's 99% certain that the control socket isn't there. In any other case there is a genuine error, and that's quite rare.