Introduce custom handler for logging facility to track number of warnings
during backup operation.
Recent changes introduce a new logging facility handler that counts the
number of warnings during backup (for example if qemu agent is not
reachable etc..)
If option "--strict" is set during backup, the exit code will now set to
2.
This allows calling applications or scripts to have a more detailed
error handling in regards to warnings and/or errors.
Full Changelog: v0.39...v0.40