disk shares saving - fixes
shares mounted on VMs are now displayed with [Virtual Mount] prefix
Merge pull request #915 from andrzej-jankowski/shares_detection_fix

Scan - shares detection
User should be forced to specify custom value for model_type when
there's only 'unknown' value for this field in db or scan results.

Empty values should be displayed as 'None' instead of an empty string.

Fixed comparison of lists in diff_results.
Merge pull request #916 from xor-xor/fix_1735

Fixes related to 'unknown' model_type and empty values in scan results form.