[libvirt] [RFC Patch 0/3]virsh: Enable env support for virsh logging

Defining environment variables for debug log-level and log-file will avoid specifying the same in each virsh command. Following is a patchset for enabling env variable support for virsh logging. Two new environment variables are defined: a. VIRSH_DEBUG=<log_level> log_level can be a value between 0 - 4, where 0 -> "ERROR" 1 -> "WARNING" 2 -> "NOTICE" 3 -> "INFO" 4 -> "DEBUG" b. VIRSH_LOG_FILE=<logfile-path-and-name> logfile-path-and-name is the name and complete path for the log file. The above two variables are independent. if log_level is specified without setting a log file name, then, logs will be displayed in stdio. If logfile-path-and-name specified without setting log_level, then, default log_level (DEBUG) will be used to log to the specified file. virsh commandline parameters (-l and -d) get precedence over environment variables and will override respective env variable values. 1/3 - Use env variables to control virsh logging 2/3 - Change log level order to make "DEBUG" the superset 3/3 - Related changes to virsh manpage