domainMonitor: Cleanup logging

- Use INFO level for the important state change message, and uppercase
  VALID and INVALID for extra visibility.
- Add few debug messages that may help debugging.
- Replace non-standard exc_info with logging.exception.
- Simplify messages text and remove unneeded words.

Change-Id: I7c24ad9f9c2c4f9b57e0682f4ab3325617b46dd9
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32505
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>