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, this is what normally happens. 'tap-ctl list' first lists all the control directory and then tries to connect to each control socket. Between these two operations, a VBD may get unplugged, therefore its tapdisk destroyed, so there will be no control socket to connect to.