CP-5961: disable DEBUG level logging in daemon mode

DEBUG level logging should only appear in the non-daemon debug mode (with "-D" flag).

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-5961: adopt blktap's standard logging facility in blktap3 new files

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>
Merge changes for CP-5961

GitHub: closes #50 on xapi-project/blktap