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>
Is ENOENT really expected to be by far the most likely cause for _connect_ to fail that we need
optimization?