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>
CA-140334: print debug message when the contol socket does not exist

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>