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

[Supriya Kannery]

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