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>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #101 on xapi-project/blktap
CA-135774: Properly log when low memory handler is not available

Use EPRINTF rather than ERR when logging if the low memory handler is not
available because it is initialized before ERR logging is set up.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #112 on xapi-project/blktap